Off da hook

rap.

Same place all those other stupid "catch phrases" came from.

Old Telephone jargon, "Busy" signal.

Old telephones actually had a little hook to hang the earpiece on when not in use.
When you didn't want to be disturbed/interrupted by an unwanted phone call, you left the phone "off the hook"...
and if you called someone and kept getting a busy signal, you'd speculate as to whether or not the phone was "off the hook".

Conversely, when there's a big buzz about something, people discuss it over the phone...
enhanced interest leads to more phonecalls, more "Busy" signals, i.e., "off the hook".

Off the hook used to mean you were getting away without punishment.

The one that got away...

fa-shizzle da bling-bling, ma bell...