Puns...Puns everywhere

A will is a legal document that designates where your assets (money, property, etc.) will go when you die. So it's a dead giveaway in that it gives away your things once you're dead. "Dead giveaway" is also a phrase for something completely obvious, like a clue that solves the whole case. So the definition of a will should be a dead giveaway - completely obvious and something that gives your things away.

The other one, "literally threw my head back", pokes fun at OP's misuse of the word "literally." One cannot literally throw [just] their head as it is attached to one's entire body.