Is there a term for words like "aftermath" or "windfall" that originally had a completely different meaning to the sense in which they are used today? Windfall for example, originally referred to fruits shaken off trees by the wind but has come to mean a stroke of good fortune. Idiomatic is the closest I could come up with but I am not sure if that is accurate.
I think this is an example of semantic shift. In the example of ‘windfall’, its meaning has widened, and it has become a metaphor for a stroke of good fortune.
There is more information about this on Wikipedia: https://en.wikipedia.org/wiki/Semantic_change
I am not aware if there is a specific term for examples of semantic shift.