flail vs beat up

flail is a noun but beat up is not a noun.

Word	Noun	Adjective	Verb	Adverb
flail	Yes	No	Yes	No
beat up	No	No	Yes	No

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

flail: give a thrashing to; beat hard
beat up: give a beating to; subject to a beating, either as a punishment or as an act of aggression

Other hypernyms of flail include beat, work over.

flail (noun)	beat up (noun)
an implement consisting of handle with a free swinging stick at the end; used in manual threshing

flail (verb)	beat up (verb)
give a thrashing to; beat hard	gather
move like a flail; thresh about	give a beating to; subject to a beating, either as a punishment or as an act of aggression