Used in 2 prominent ways:

1. Used as japanese slang for "Thank you," as 3 and 9 are "san" and "kyu" respectively.

2. Used to reference Hatsune Miku, as the alternate way of saying 3 and 9 are "mi" and "ku" and thus can be used to reference her.

a phrase chiefly used in northern England.

it is a contracted form of "it'll be reet", where "reet" means "alright" or "OK".

living your life according to "be reet" is the only way to find happiness in life.

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.

Screw 69, have you heard of 39? This number resembles anal sex.

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!!

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