Why did doubting thomas get his name?

Doubting Thomas is so called because unlike the other apostles, he refused to believe Jesus had risen from the dead without solid proof in front of him own eyes. He could not have faith with out proof. Jesus appeared to some of the disciples, but Thomas was not with them. When the other disciples told him what had happened he rebuked them and said " Unless I see the nail marks in his hands and put my finger where the nails were, and put my hand into his side, I will not believe it." (John 20:25) Eight days later Jesus appeared to the disciples again and this time Thomas was with them. He stood among them and said "Peace be with you" and then turning to Thomas he said, "Put your finger here; see my hands. Reach out your hand and put it into my side. Doubt no longer but believe" (John 20:26-29). Thomas then said that because he has seen he could believe. Jesus said to him "You believe because you can see me. Happy are those who have not seen and yet believe".

This is where the name Doubting Thomas comes from.