pen up vs restrain

pen up and restrain both are verbs.

Word	Noun	Adjective	Verb	Adverb
pen up	No	No	Yes	No
restrain	No	No	Yes	No

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

pen up: confine in a fold, like sheep
restrain: to close within bounds, or otherwise limit or deprive of free movement

Other hypernyms of pen up include confine, constrain, hold.

pen up (verb)	restrain (verb)
confine in a fold, like sheep	prevent the action or expression of
	to close within bounds, or otherwise limit or deprive of free movement
	restrict (someone or something) so as to make free movement difficult
	prevent (someone or something) from doing something