couple vs pair off

couple is a noun but pair off is not a noun.

Word	Noun	Adjective	Verb	Adverb
couple	Yes	No	Yes	No
pair off	No	No	Yes	No

As verbs, couple and pair off are synonyms defined as:

Other synonyms of couple include pair, partner off.

couple (noun)	pair off (noun)
a pair who associate with one another
a pair of people who live together
(physics) something joined by two equal and opposite forces that act along parallel lines
two items of the same kind
a small indefinite number

couple (verb)	pair off (verb)
engage in sexual intercourse	form a pair or pairs
link together
bring two objects, ideas, or people together
form a pair or pairs