WordReference Random House Unabridged Dictionary of American English © 2024doubt′ing Thom′as, - Biblea person who refuses to believe without proof;
skeptic. John 20:24–29.
Collins Concise English Dictionary © HarperCollins Publishers::
doubting Thomas n - a person who insists on proof before he will believe anything; sceptic
Etymology: after Thomas (the apostle), who did not believe that Jesus had been resurrected until he had proof