Wiktionary
n. (context idiomatic English) Something that discloses, usually unintentionally, a fact or an intention.
Usage examples of "dead giveaway".
Leaving him a note would be a dead giveaway if someone broke in, and he was too smart to answer the phone if I called just after dark had fallen.
The eyes are obvious, but my ears and hair are a dead giveaway from a distance.
It would have been a dead giveaway for him to ignore his master's orders.
The three of you marching purposefully on Gaza will be a dead giveaway.
She thought it would be a dead giveaway, but when he took out the clean set of sheets, they were identical to the ones that had just come off.
I'd left the electric fire burning in my room, the communicating door was open and I was pretty sure it was only a matter of time till she could see enough of my features to see that I wasn't Jablonsky -- and that red thatch of mine was a dead giveaway.
He had to have the operation running perfectly along the standard track, a dead giveaway that he was concerned more with the impression he generated within the company than with the practicalities of the situation they'd be facing.
The light plaster walls and ceiling showed off blood and caked-on brains to full advantage, the teeth fragments and buckshot a dead giveaway that the victim had stuck both barrels in his mouth.
There were a lot of business trips after that, usually at weekends, which I thought was a dead giveaway.