from Wiktionary, Creative Commons Attribution/Share-Alike License
- adv. Of the cue ball: (going) into a pocket, after cannoning another ball.
- n. When the cue ball accidentally goes into a pocket after striking the object ball.
From a reconstitution of (go) in + off (preposition), as used in phrases such as, "The white has gone in off the black." (Wiktionary)