pen up vs constrain

pen up and constrain both are verbs.

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

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

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

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

pen up (verb)	constrain (verb)
confine in a fold, like sheep	severely restrict in scope or extent
	to close within bounds, or otherwise limit or deprive of free movement
	compel to behave in a certain way