do it vs pair

do it is not a noun while pair is a noun.

Word	Noun	Adjective	Verb	Adverb
do it	No	No	Yes	No
pair	Yes	No	Yes	No

As verbs, pair is a hypernym of do it; that is, pair is a word with a broader meaning than do it:

Other hypernyms of do it include copulate, couple, mate.

do it (noun)	pair (noun)
	two people considered as a unit
	a set of two similar things considered as a unit
	a poker hand with 2 cards of the same value
	two items of the same kind

do it (verb)	pair (verb)
have sexual intercourse with	engage in sexual intercourse
	bring two objects, ideas, or people together
	arrange in pairs
	occur in pairs
	form a pair or pairs