(informal, slang, Canada, US) an exclamation of excitement or joy

(colloquial) Yes; an expression of affirmation.

(informal, vulgar, colloquial) Expressing joy, celebration, glee, support, elation, etc. / (informal, vulgar, colloquial) A strong affirmative response.

(Australia, New Zealand) A polite expression of either agreement or disagreement (clarified by following speech): can be used to denote conclusion of cursory difference(s) of opinion between speakers ("yeah-nah, he's a top bloke at the end of the day"), and/or perceived escalations of discord ("yeah-nah, I can, however, see where you're coming from").

Alternative form of yeah, nah

(Ireland, sarcastic) I will not.

(slang) Used to express enthusiastic or gung-ho agreement or joy.