understock vs stock up

understock and stock up both are verbs.

Word	Noun	Adjective	Verb	Adverb
understock	No	No	Yes	No
stock up	No	No	Yes	No

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

understock: stock with less than the usual or desirable number or quantity
stock up: amass so as to keep for future use or sale or for a particular occasion or use

Other hypernyms of understock include buy in, stock.

understock (verb)	stock up (verb)
stock with less than the usual or desirable number or quantity	amass so as to keep for future use or sale or for a particular occasion or use