Thumb in the Can

A phrase used to describe the finality of a decision made by someone.

Once someone has made a decision and can not back out of it.

Derives from the immature act of sticking one's thumb in a can and attempting to pull it back out.

Jim: Why don't you just leave her?
Ted: I can't mate, I've got my thumb in the can now.

Wish I'd have said no to that promotion...now I've got my thumb in the can.