doubt ‧ ing Thom ‧ as /ˌdaʊtɪŋ ˈtɒməs $ -ˈtɑːm-/ BrE AmE noun [singular]
[ Date: 1800-1900 ; Origin: Thomas the follower of Christ who did not believe he had come back to life ]
someone who does not always believe things until they have seen proof of them. This expression comes from the story of the ↑ apostle Thomas in the Bible. He doubted the truth of the ↑ resurrection (=return to life) of Jesus Christ and demanded to feel Jesus' wounds before he would believe it.