This expression's meaning may be guessed from the words in it, so it is not an idiom. To play possum is to deceive an attacker by pretending to be dead or vanquished, as the possum proverbially does.
It is actually an idiom.
The opening line of the book, "Old Marley was dead as a doornail." Dead as a doornail is an idiom.
Nothing
Dead to the world means sleeping soundly. One example of a sentence with the idiom dead to the word is: After working a double shift, Sarah fell asleep and was dead to the world.
In one word: dead.
The meaning of the idiom dead certainty is absolutely or definitely
The idiom is "dead certainty". It means it's in the bag = this is a dead certainty = this is cinch
I wouldn't be caught dead in sleeping during the class.
You see, dead people are really hard to wake up. Because they're dead.
This is not an idiom. Idioms make little or no sense unless you know the definition. This sentence makes perfect sense, so it is not an idiom. The dead fish smelled so bad that even as high as Heaven, you could smell them.
One idiom for flowers is "pushing up daisies". This means to be dead or buried. This originated when people put daisies on graves in cemeteries.