knuckle under vs succumb

knuckle under and succumb both are verbs.

Word	Noun	Adjective	Verb	Adverb
knuckle under	No	No	Yes	No
succumb	No	No	Yes	No

As verbs, knuckle under and succumb are synonyms defined as:

Other synonyms of knuckle under include buckle under, give in, yield.

knuckle under (verb)	succumb (verb)
consent reluctantly	consent reluctantly
	be fatally overwhelmed