chat up vs flirt

chat up is not a noun while flirt is a noun.

Word	Noun	Adjective	Verb	Adverb
chat up	No	No	Yes	No
flirt	Yes	No	Yes	No

As verbs, chat up and flirt are synonyms defined as:

Other synonyms of chat up include butterfly, coquet, coquette, dally, mash, philander, romance.

chat up (noun)	flirt (noun)
	playful behavior intended to arouse sexual interest
	a seductive woman who uses her sex appeal to exploit men

chat up (verb)	flirt (verb)
talk or behave amorously, without serious intentions	talk or behave amorously, without serious intentions
talk to someone with the aim of persuading him	behave carelessly or indifferently