idle talk vs yak

idle talk and yak both are nouns.

Word	Noun	Adjective	Verb	Adverb
idle talk	Yes	No	No	No
yak	Yes	No	Yes	No

As nouns, yak is a hypernym of idle talk; that is, yak is a word with a broader meaning than idle talk:

Other hypernyms of idle talk include cackle, chatter, yack, yakety-yak.

idle talk (noun)	yak (noun)
idle or foolish and irrelevant talk	large long-haired wild ox of Tibet often domesticated
	noisy talk

idle talk (verb)	yak (verb)
	talk profusely