Ok we know what a 69 is right? if not, GTFO here!. A 6699 is a four way orgy, Where the two partners in the middle are doing a standard "eat eachother out",(aka 69), and the eaxtra "6" and extra "9" are butt fucking there corresponding numbers.
Dave: Had a sweet 6699 last night.
Eli: What the fuck is a "6699".
Dave: Ok lemme explain little Eli, A 6699 is a four way orgy where these two people 69'd and me n' this other dude buttfucked the two people 69'ing.
Eli: So who was sixty nining?
Dave: Some dude and some Chick, I dont remember their names.
Eli: So one of you fucked a dudes ass
Dave: .......Fuck(he sais under his breath)