coup de main vs onset

coup de main and onset both are nouns.

Word	Noun	Adjective	Verb	Adverb
coup de main	Yes	No	No	No
onset	Yes	No	No	No

As nouns, onset is a hypernym of coup de main; that is, onset is a word with a broader meaning than coup de main:

Other hypernyms of coup de main include attack, onrush, onslaught.

coup de main (noun)	onset (noun)
an attack without warning	(military) an offensive against an enemy (using weapons)
	the beginning or early stages