I'm not being funny

does the Pope wear a funny hat

Obviously! Rhetorical question in response to a question where the answer is an emphatic yes.

funny bone

The ulnar nerve in the elbow, which, when hit, causes a tingling sensation.

funny man

Alternative form of funnyman.

funny stuff

Irregular, often illegal, activities.

I'm not telling you

Used to express that someone doesn't need to know something, typically because it is a secret (sometimes used as a way to attempt to hide someone's wrongdoing); may also be used when someone tries to ask for answers while doing an assignment or examination, or private or confidential information, including passwords.

I'm not the one

A warning to an individual who has greatly offended one's dignity or has acted or spoken insolently.

I'm sure

Used as an interjection to indicate denial or contradiction in response to an assertion or accusation.

if I'm honest

to be honest