Basically, it's "fuck," but fabulous! Not to mention generally cleaner and more sensitive to others.
Origin - Some conservative wannabe faggot went on a homophobic rampage against the California Protection of Marriage Act (the satirical one that bans divorce to save marriage from the straights) and his last name was Foong. So we told him to foong off and turned his name into what he hates. Win.
And don't forget "foongy," for someone with that especially tight ass. Oo baby! Work it~
"I hear Ted Haggard likes a good foonging after smoking crystal meth."
"You can foong me, or I can foong you. All that matters is that we foong loong and hard."
"You're soooo fucking foongy... Gimme some of that sugar, sweetcheeks!"