To block, suppress, restrain.
The chairman sat on the report until the end of the legislative session.
To restrain (a person).
He started to act up, but she sat on him.
To take no action on.
I sent the boss my proposal three weeks ago and he's been just sitting on it.
To be a member of.
He sits on the appropriations committee.
To hold an official inquiry regarding; to deliberate about.
To have but not actively use