poleax vs strike down

poleax is a noun but strike down is not a noun.

Word	Noun	Adjective	Verb	Adverb
poleax	Yes	No	Yes	No
strike down	No	No	Yes	No

As verbs, strike down is a hypernym of poleax; that is, strike down is a word with a broader meaning than poleax:

Other hypernyms of poleax include cut down, drop, fell.

poleax (noun)	strike down (noun)
a battle ax used in the Middle Ages; a long handled ax and a pick
an ax used to slaughter cattle; has a hammer opposite the blade

poleax (verb)	strike down (verb)
fell with or as if with a poleax	cause to fall by or as if by delivering a blow
	cause to die, especially suddenly
	declare null and void; make ineffective