"Didymus" means "twin" in Greek, which is also the meaning of "Thomas" in Aramaic. Thomas seems to have been of a somewhat cynical turn of mind (John 14:5; 20:25) until after He had seen Jesus alive from the dead (John 20:28).

Source:

KJV Defenders Study Bible, by Dr. Henry Morris, Ph.D.
Publisher: Thomas Nelson
Language: English
DEF 10 ISBN 0-529-10444-x
DEF 10-1 ISBN 0-529-10445-8