off the hook · adjective1
An allusion to a fish caught on the hook of a fishing line.
off the hook · adjective2
Referring to the candlestick phone, which in order to hang up, the receiver must be hung on a hook. Leaving the receiver off the hook would prevent it from being able to receive calls.
off the hook · adjective3
Unknown.
Etymology from Wiktionary, CC BY-SA 4.0.