shake (hands) on something
to clasp and shake the hand of someone as a sign of agreement about something. The two people didn't sign a contract; they just shook hands on the terms of the agreement. I think it would be better to sign an agreement than just shake on it.
To make some agreement binding by shaking hands: Our negotiations went well, and we soon shook on the deal. If you're satisfied with the arrangement, let's shake on it.