verb/de.fɔ̃.se/
to deep-plough; to smash in, smash up, break, break open (destroy violently); to rip, rip apart, tear apart; to fuck up, make high, screw up (intoxicate); to get high, get pissed
“J'ai hâte de pouvoir lui défoncer son mignon cul.” — I can't wait to fuck his cute ass.
From dé- + foncer.