Bust a NUT means to ejaculate. Typically for men, but more and more women are using the term too. Bust a NUT is also referred to as cumming, cum, orgasm, climax, cream, jizz, emit semen, shoot one's load, drop a load, spurt, squirt, shoot one's wad, get your rocks off, etc.
Dude. I can't wait to get home, watch some porn, and Bust a NUT. I need to drain this nutsack badly.