The intense headrush and subsequent complete knockout punch that comes immediately after smoking a gigantic rip of marijuana out of a bong. Typically the person experiencing bongface is incapable of speech or even thought for about one minute, after which the high disperses throughout the body, and the user becomes pleasantly blazed.
Person 1: /takes massive bong hit
Person 2: Dude, you're about to get the craziest bongface.
Person 1: /incoherent speech while closing eyes tightly and putting head in arms