The phrase "accident waiting to happen" is an example of what type of figure of speech?

The story behind the answer

“Accident waiting to happen” is an idiom—a fixed expression whose meaning isn’t literal. It doesn’t describe a person or object that is literally an accident; it figuratively warns that a situation is obviously risky and likely to lead to trouble. It isn’t a simile (no “like/as”), nor a straightforward metaphor naming one thing as another in a poetic way, nor an analogy explaining a relationship between two pairs.

Idioms are common, culture-shaped shortcuts in language; their meanings can’t be reliably guessed from individual words. Memory tip: if a familiar, set phrase can’t be taken literally yet is widely understood (“spill the beans,” “break the ice”), it’s an idiom—like “an accident waiting to happen,” used to flag danger before it occurs.