forearm vs build up

forearm is a noun but build up is not a noun.

Word	Noun	Adjective	Verb	Adverb
forearm	Yes	No	Yes	No
build up	No	No	Yes	No

As verbs, build up is a hypernym of forearm; that is, build up is a word with a broader meaning than forearm:

Other hypernyms of forearm include arm, fortify, gird.

forearm (noun)	build up (noun)
the part of the superior limb between the elbow and the wrist

forearm (verb)	build up (verb)
arm in advance of a confrontation	form or accumulate steadily
	enlarge, develop, or increase by degrees or in stages
	change the use of and make available or usable
	bolster or strengthen
	prepare oneself for a military confrontation