Where does 69 come from?
Vive la France! The slang 69 goes back, if you can believe it, to the French Revolution. The term is found in a 1790 French work, The Whore's Catechisms, attributed the revolutionary figure Théroigne de Méricour, who described a soixante-neuf, or "sixty-nine" in French.
English picked up sixty-line by the 1880s, with its numerical shorthand, 69, familiar by the 1970s.
As long as there have been humans, though, there probably has been the practice of a 69. It's described in the ancient Indian Kama Sutra, for instance, as the Congress of a Crow position. The position involves different- or same-sex partners going down on each other—genitals or anus—at the same time, usually achieved by lying on their sides or one on top. That is unless the fornicators take to the Standing 69 or Eiffel 69, known variants of the move.
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69 is commonly referenced—and snickered about—in popular culture. R&B singer Rick James released "She Blew My Mind (69 Times)" in 1982, for instance, and Bryan Adams 1984 "Summer of '69," which puns on the peace-n-lovin' year 1969. Rapper Daniel Hernandez is better known by his suggestive stage name, 6ix9ine.
In in the 2000s, 69 = inspired something of a meme where internet users comment "nice" on just about any online content that happens to feature the number 69 as such (e.g. the poll shows that 69% of Americans approve of the new law).
Memebase - Cheezburger
Not that this is new. Pretty much all of us who are familiar with 69 make jokes whenever we encounter 69 in the wild, from phone numbers to jersey numbers to birthdays to license plates.
Who uses 69?
69 is an adult, NSFW term, but it isn't as offensive as a word like fuck so it can't be that bad.
Outside of literal uses (including as a verb, to sixty-nine someone), 69 is used throughout the sex industry, from pornographic Twitter handles to sex to phone-sex telephone numbers, as a shorthand for anything "sex."
Nice is not the same as kind. / Nice is not the same as ethical. / Nice is not the same as moral. / Nice is the sex number; it is the number 69.
@engeej, May, 2018
The classic oral sex position but, instead of one of you on top of the other, try it with both of you laying on your side...Some people find 69 lackluster because there's too much going on, but this variation is more grounded.
Lea Rose Emery, Bustle, May, 2018
[I]t's really hard for me to get past the idea that that is “the true meaning of 69.” 6ix9ine has gotten famous so perhaps he feels that he’s more of an authority on 69 than the rest of us, but I’m here to say that “the true meaning of 69″ is sex.
Jordan Sargent, Spin, March, 2018