Idiomatic To take no action on (something); to hold (something) in reserve without actually using.
I sent the boss my proposal three weeks ago and he's just been sitting on it.
I began to call the INS regularly to check up on the status of my application. I got the proverbial "run-around," and this effort only confirmed my suspicion that the INS was sitting on my application.1986, Gilberto Gerald, “With My Head Held Up High”, in Joseph Beam, editor, In The Life: A Black Gay Anthology:
4
To be a member of (something).
He sits on the appropriations committee.
5
To hold an official inquiry regarding (something); to deliberate about (something).
6
Euphemistic To insert (something) into one's vagina or anus from below.
Let me sit on it.
7
Used other than figuratively or idiomatically: see sit, on.