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.

to take an unfortunate loss and turn of events.

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 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 39 is not an ordinary party its where people get f*cked up beyond self control but the vibes are always good and having fun is what's important

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