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69 or Sixty-nine is a slang term and memetic number used to refer to a sex position due to the number looking like two people having mutual oral sex. Online, it has a reputation as "the sex number," and has been featured in numerous memes and jokes over the years. People often joke about it by replying "nice" to posts that use the number 69 regardless of the context in which the number is used.
History as a Sexual Term
The earliest evidence of people participating in the sexual position was in France in the 1790s. Soon after the position became known, the term soixante-neuf (French for sixty-nine, hence the number) was given to it. The earliest known use appeared in Whore's Catechisms in France. The Kama Sutra refers to the position as "The Congress of a Crow."
The term began appearing in American pop culture in the mid-late 20th century. Notably, two pop songs, in particular, use the number to refer to the sex position. The first, "She Blew My Mind (69 Times)," was composed by R&B singer Rick James in 1982. The chorus of the song goes "She blew my mind 69 times," a reference to "blowing" being slang for oral sex and the number being a reference to the position (shown below, left). The other, more controversially, is Bryan Adams' "Summer of '69," composed in 1984 (shown below, right). While his co-writer, Jim Vallance, denies rumors that the song is a reference to the sex position, Adams explicitly states it is about the sex position in an interview cited by Uproxx.
I think [“Summer of ’69” is] timeless because it’s about making love in the summertime. There is a slight misconception it’s about a year, but it’s not. “69” has nothing to do about a year, it has to do with a sexual position…At the end of the song, the lyric says that it’s me and my baby in a 69. You’d have to be pretty thick in the ears if you couldn’t get that lyric.
In the 1989 comedy film Bill & Ted's Excellent Adventure, the titular characters choose "69," when asked to randomly pick a number that would prove their future selves were truly them (shown below).
It is unknown when exactly the first use of the number 69 as a slang term for the sexual position appeared online, however, it was prevalent dating back to at least the early 2000s. On December 8th, 2003, Urban Dictionary user starflier uploaded the commonly understood definition of the number, gaining over 20,600 upvotes in 14 years.
Simultaneous oral sex. Can be practised by M+M, M+F or F+F. The reference is that the numerals 6 and 9 are inverted versions of each other, reflecting the physical position that the couple must adopt when engaged in this sexual practice.
Usage and Association with "Nice"
Online, it is a common practice to reply to every post with the number 69 in it, regardless of context, with "nice." In June 2017, The Daily Dot published an extensive history of the practice. According to its research, the words "69" and "nice" began to be commonly associated together in the summer of 2008 (examples shown below).
The Daily Dot notes that the practice of saying "nice" after 69 likely originates from an episode of South Park called "Miss Teacher Bangs a Boy" in which the cops respond to claims that a female teacher is having sex with a male toddler with, "Nice" (seen below).
Prominent early examples of "69" being associated with "nice" came in 2009 with regard to sporting events. For example, during a football game, commentator Al Michaels mentioned how Vikings defender Jared Allen was harassing quarterback Kurt Warner by saying, "Sooner or later, #69 will be in your face." Twitter user Playstub replied to the quote with "nice" (shown below).
In the following years, it would become practice to mass-reply to any tweet or comment around the internet with the number in it, regardless of context, with "nice." For example, when Barack Obama tweeted that 69 percent of Americans wanted hearings for his Supreme Court nominee pick Merrick Garland on May 13th, 2016, the primary response was "nice" (examples shown below).
Trump's "69 / Nice" Tweet
People quickly noticed that Trump had used both "69" and "nice" in the same tweet, leading to some stunned reactions as people wondered whether Trump had intentionally used the joke. Twitter user @SonOfGodandMan, a Jesus Christ parody account, called it Trump's "best tweet ever," gaining over 170 likes (shown below, left). User @itsjefftiedrich posted "oh god oh god oh god oh god oh god he didn't did he," gaining over 240 likes (shown below, right).
Others expressed dismay that the use of "69" and "nice" was likely unintentional and could have the unintended consequence of ruining the joke. Reporter Ashley Feinberg tweeted "folks i'm sorry, 69 is cancelled," gaining over 60 retweets and 740 likes in roughly half an hour (shown below, left). User @goldengateblond tweeted "farewell 69, you had a good run" (shown below, right). The tweet and reactions were covered by several media outlets such as Indy100 and Mashable.
Type YY Into Google
Type YY Into Google refers to an interactive joke which required participants to search the Chinese company YY in the Google search engine between 4 p.m. on June 28th, and 9:30 a.m. EST on July 1st, 2019. The inquiry produced a panel with the company's share price as of the stock market closing time on June 28th, which was at 69.69 USD; a double "sex number."
 The Daily Dot – The complete history of the ‘69-nice’ meme on Twitter