(slang, Britain, West Country) isn't

Used to add annoyed emphasis to an associated statement.

Used other than figuratively or idiomatically: see sure. / (usually sarcastic) Used to emphasise an apology, acknowledgement, etc.

(vulgar) Expression of surprise or irritation

(fandom slang) Used to imply that one needs to excuse oneself to privately indulge in a pleasurable activity, particularly masturbation.

(vulgar, colloquial) An expression of astonishment or disbelief.

I will let you be in charge; you can make the decisions or choices.

(idiomatic, colloquial) Synonym of I can tell you

Used before stating something that the speaker believes to be correct

(colloquial) Synonym of I can tell you