A Hebrew name meaning twin.

It became a popular name in countries influenced by Christianity because it was the name of one of the Apostles known for his inquisitive mind.

He was also quite a missionary as he traveled farther than any of the other Apostles. He certainly made it as far as India, but according to a legend he came as far East as America (most likely just a legend though).

Because he traveled so far, the Christian communities that he started did not stay in touch with the rest of Christianity. This presented an interesting challenge to the Catholic Church centuries later after first contact was made: The Thomas Christians were not under the authority of the Pope. Not because they split, but because they never heard of the Pope. Yet, since they were started directly by one of Apostles they could not be considered schismatics, let alone heretics.

Because of that, the Catholic Church had no choice but to accept the Thomas Christians as being a fully legitimate Christian Church, perfectly equal in all respects to the Catholic Church. While the Catholic Church views all other non-Catholic Christians as either schismatics or heretics (and, hence, "blemished" one way or another), to this day it accepts Thomas Christians as being neither.

By the way, chances are Thomas was not the Apostle's actual given name. He was probably a twin and hence nicknamed Thomas.