The Bible and the Talmud do not contain any reference to this prohibition. Indeed, just the opposite, the Lubavitcher Rebbe notes that from verses in Genesis it is evident that Terach (father of Abraham) named his son Nachor during the lifetime of his father Nachor.(See Genesis 11:24-26; Sha'arai Halacha U’Minhag,Yoreh Deah, Volume III, p.298)
In addition, the Talmud records a case of a mother concerned about the circumcision of her third son whose two older son’s died as a result of circumcision. Rav Natan gave sage advice that was followed and the child lived and was named “Natan HaBavli”. (Shabbat 134a) The overt indication appears to be that the name given to the child after the Rav was an act of honor to the Rav and certainly not a sinful act.
The Talmud, however, does not grant total discretion to parents to name children any name they so select. The one limitation was a rabbinic ruling prohibiting to name children with names of sinners or evildoers (R’sha’im).(Yoma 38b)This rule has yielded two basic interpretations A simple understanding is that this is a practical means of expunging the names of Jewish sinners from usage. The implication is that there is a general ban against mentioning the names of the wicked and the practical vehicle to curtail such usage was to simply not name children after them. Rabainu Chan’nanel notes that the prohibition against using the names of the wicked means that “any person so named will not be successful”.This implies a form of a curse. Any child named with the name of an evildoer will simply not be successful in life. What parent would wish to jeopardize the success of a child by disregarding the rabbinical ban? There is no question as to whether parents will refuse to name children the names of sinners once they are aware of the projected doom for anyone so named.The uniqueness of Rabbainu Cha’anel’s position is that he introduced elements of mysticism and fear into the ordinary function of naming a child.
In Ashkenazic Europe the custom developed to refrain from naming children with the names of living persons. The following rationales are presented.(Some with sources, some without.).
Common custom is to name children after parents or grandparents who are no longer alive. To name a child after a living person gives the impression that one wishes they were dead, Chas V’Shalom.- (B’rit Avot 8:20 cited in the name of Noheig Katzon Yosef)
When a child, together with his/her father or grandfather have the same name, the Angel of Death may, by mistake, kill the youngest rather than the father or grandparent.
According to Jewish law it is not deemed proper respect to call one’s parent by his/her first name.(Yoreh Deah 240:2) Giving a child the name of the living parent or grandparent would generate confusion and a belittlement of respect.(Chelkat Yaakov, Yoreh Deah 136,Shmirat HaGuf V’haNefesh, Vol. II, 154:9)
To forestall such errors, Ashkenazim simply did not name children after a living person. Thus, concern for proper respect for parents, mysticism, coupled with fear of the “evil eye”, serve as the basis for the custom. There never was an official rabbinic law to outlaw naming a child after a living person. It is merely a custom .that has prevailed comparable to a rabbinical ban.(It is merely an extension of the mystic position of Rabainu Chan’anel.)
Many years ago a family requested that I perform a wedding during the Nine Days commemorating the destruction of the holy Bet HaMikdash. I mentioned that according to jewish law one was not to be married during this period of time. To this they responded that they were not too religious and were not perturbed about violating the law. When I mentioned that it was deemed “Bad Luck” to get married at that time, they immediately changed the date for the wedding. In other words even Jews who are not observant on a regular basis will not be involved with any matter shadowed by the spectre of “Bad Luck”. So too with the Ashkenazic ban against naming a child after living persons. No one wishes to galvanize “Bad Luck” upon their children.(Kashe sakanta m’isurah)
The Sefardim simply never adopted any such customs. They follow the original tradition wherein it was totally permitted to name children after living persons. Indeed, they deem the act as a form of granting honor to parents or grandparents.
It has become an Ashkenazi custom to name a new baby after a relative that has died. The idea behind this custom is it keeps the name and memory alive, and in a metaphysical way forms a bond between the soul of the baby and the deceased relative. However, it is by no means forbidden to give a child the same name as a living relative. The living relative should be asked permission. The reverse is true in Sephardic communities - the custom is to give the child the name of a living relative, often a grandparent. This custom is based on a desire to honor the living, while they are still here and give the child a sense of the qualities in real life the parents' are hoping he/she will live up to. However, this too is custom and not law and if you choose to name a child after a deceased relative in a Sephardic family you might want to give family members accustomed to a different custom time to process the difference.
It is however, all custom and as such parents should take great thought and care in naming their child. In many way this is the beginning of our identities, our names shape how we see ourselves and how others see us. Often choosing a name is challenge, in Judaism in particular this is seen,much like Adam names the animals in the garden, as our sharing in the creation of a human being with God. So we think of qualities we want in our children, we think of relatives and friends who have impacted our lives and their honor and we think of the child and who we hope they become - naming is an important aspect of the early stages of raising a Jewish child so whatever your custom is or whatever custom you decide to follow think carefully and seriously about the importance of your role as a "namer, creator" of this child! Happy hunting.
The practices in both Ashkenazic and Sephardic communities around naming are based in custom. Both are customs based on superstition surrounding the Angel of Death. In Ahskenazic communities it was believed that if you named a baby after a living person, when the Angel of Death came for the older person, he would become confused and take the younger one instead. In Sephardic communities they took the opposite approach believing that if you named a baby after a living person, the Angel of Death would become confused and take neither the older nor the younger person.- therefore there was a benefit to those for whom babies were name.
While Reform Judaism tends to stay away from superstition, the practice of naming after a deceased relative (and specifically not living ones) is still practiced by the vast majority of Reform Jews out of a sense of tradition and a desire to honor those who have passed on prior to a new life coming into the world.With that said, the naming customs in the Reform movement tend to be based on the ancestry of the family, as the small minority of Reform Jews with Sephardic backgrounds will name after living relatives.