yack vs speak

yack is a noun but speak is not a noun.

Word	Noun	Adjective	Verb	Adverb
yack	Yes	No	Yes	No
speak	No	No	Yes	No

As verbs, speak is a hypernym of yack; that is, speak is a word with a broader meaning than yack:

Other hypernyms of yack include mouth, talk, utter, verbalise, verbalize.

yack (noun)	speak (noun)
noisy talk

yack (verb)	speak (verb)
talk incessantly and tiresomely	express in speech
	exchange thoughts; talk with
	use language
	give a speech to
	make a characteristic or natural sound