wouldn't you know

as you know

as it is already known.

butter wouldn't melt in someone's mouth

The identified person appears to be benign, mild-mannered, or calm (but with an integral suggestion that, to the contrary, they really are untrustworthy, disagreeable, or mean-spirited).

fuck if I know

I have no idea whatsoever

hell if I know

Synonym of I don't know.

I don't know about that

I am not sure that I agree; expressing doubt or skepticism.

I hardly know her

A joking retort that draws attention to the possibility of sexual innuendo when a word ends with a schwar.

it takes one to know one

Expressing the same underlying character-judgment themes as pot calling the kettle black or set a thief to catch a thief.

know beans about

To know nothing, or almost nothing, about.