1. To be held responsible (for something); to be obliged or committed (to something). I would think very carefully signing your name to those documents. If something goes wrong in the company, it could be you who's on the hook. I'm on the hook for nearly half a million dollars after my partner skipped town with the company's pension fund!

2. To be in a continually and/or indefinitely delayed condition or state. The suspect has been on the hook in the interrogation room for about 12 hours now. We're keeping him guessing in the hopes that he'll let some information slip.