a dead giveaway
noun

- something that shows (the truth about something) clearly

Merriam-Webster

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WORD ORIGIN

The American Heritage Dictionary of the English Language says a giveaway, apart from being a gift, is something that accidentally exposes or betrays.

Another way to describe it would be “an unintentional disclosure”. It may therefore seem odd that by adding “dead”, it takes on a slightly different meaning.

Dead is used in a vast number of figurative senses however, many of which are used for emphasis or to express a higher level of intensity. Thus a dead giveaway is more than just a giveaway, it’s an obvious one.

As an adjective, dead gets lots of use. There is the sense of no longer active, such as a dead volcano (also called dormant), or not having the capacity to sustain life like dead soil.

Things that cease to function can also be referred to as dead; a dead tennis ball (no longer bounces properly), dead coals or a dead flame (extinguished), dead motor or a dead battery. A ship that can no longer be controlled is referred to as “dead in the water”.

Dead is also used to emphasise the period when something is most intense: in the dead of night, in the dead of winter.

Or it can be used to describe something that is lacking excitement or activity like a dead town (no apparent human activity) or a dead party (dull, boring).

Dead is also applied in the sense of suddenly (he came to a dead stop, he stopped dead in his tracks), complete (dead silence), exact (dead centre), directly (dead ahead) or certainty (dead sure, dead certain).

And if someone speaks of a dead issue, then it’s a problem or discussion item that is no longer relevant.

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SYNONYMS

betrayal, communication leak, exposure of secret, giveaway, leak, telltale, telltale sign

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SMUGGLE OWAD into a conversation
say something like:

“The first slide of his presentation was A DEAD GIVEAWAY that he wanted to take control of the department.”