Japanese style of "Thank You"
3 = San (spelling "sun")
9 = Kyu
39 = SanKyu = Thank You

In Japanese, 3 is "san" and 9 is "kyuu".
In a Japanese accent, "Thank you" sounds like "Sankyuu", people will often use "39" as a way of saying thanks.

The act of doggy style, hitting it from the back ferociously, sliding up to the bumper in order to saddle up and ride. Other variations include pterodactyl and is based on doggy style. Phallus can be inserted into anus or vagina. True 39ing normally includes a great deal of screaming. Blood flow may ensue. 39ing never includes homosexuals queer(s) and faggot(s).

the age before 40 and the beginning of the end. It's all down hill from here, girlfriend! Time to put it in reverse and back that ass up!!

A fat dumb asian kid hu gets 39/40 in a test but says 3.9 in an Korean accent

Emoticon that denotes anilingus. Turned on its side, the 3 becomes butt-cheeks, the 9 becomes a smiling mouth with tongue.

A woman who is a 3/10 in real life but when she is in the atmosphere of a nightclub she has the attitude and style of an attractive woman. They often reject guys who want to dance or buy them a drink.