I had never heard of this until last year, but suddenly everyone on the internet is using it. I was wondering where it came from and why it took off so quickly.
She wants the D.
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According to knowyourmeme:
“The D” first made an appearance on the 2004 single “So Sexy” by American rapper Twista featuring R&B artist R. Kelly.
The entry for "the D" on Urban Dictionary was created in 2004 as well.
Though the use of the term became more widespread through Tumblr in 2012:
...following the viral spread of Give Her the Dick, an oddly phrased comment that was originally posted on the adult website Pornhub
Throughout 2012, the phrase “she wants the D” was mentioned in dozens of dating and relationship advice threads on Yahoo Answers and the Bodybuilding Forums.