pep up vs cheer

pep up is not a noun while cheer is a noun.

Word	Noun	Adjective	Verb	Adverb
pep up	No	No	Yes	No
cheer	Yes	No	Yes	No

As verbs, pep up and cheer are synonyms defined as:

Other synonyms of pep up include barrack, exhort, inspire, root on, urge, urge on.

pep up (noun)	cheer (noun)
	the quality of being cheerful and dispelling gloom
	a cry or shout of approval

pep up (verb)	cheer (verb)
make more interesting or lively	become cheerful
spur on or encourage especially by cheers and shouts	spur on or encourage especially by cheers and shouts
	show approval or good wishes by shouting
	cause (somebody) to feel happier or more cheerful
	give encouragement to