Origin of dead giveaway [duplicate]
From http://www.phrases.org.uk/bulletin_board/56/messages/494.html:
"Giveaway" in the sense "inadvertent betrayal or revelation; something that gives [the game] away" is US slang dating from 1882. "Dead" has here the sense "absolute, sure", which was used in many phrases coined around that time - e.g. dead ringer, dead centre, dead earnest, dead certainty. (VSD)