Dead Giveaway Meaning
(idiomatic) Something that discloses, usually unintentionally, a fact or an intention.
Example: That disguise is a dead giveaway! Choose something less obvious.
2008, New York Times, March 5
"Instead of slowing down his arm when he throws it â€” a dead giveaway to hitters â€” Igawa had better arm action, Eiland said"