Slang, usually past-tense, for ejaculation. Many say it originated with fans of the Foo-Fighters (possibly may have even started with Nirvana) probably in reference to the expression of frontman Dave Grohl when performing, as his intensity resembles that of a person having an orgasm. While this origin is disputed, many say the word just sounds appropriate for the act, which most likely explains why it caught on.
Dude, I can't believe it! That stripper took me in for a private dance, and I freaking grolled in my pants!