Don’t worry, he knows refers to one of the final paragraphs in a popular 4chan greentext story.
The story is written by a teenage user, who describes the process of his “pseudo-girlfriend” being seduced by the older cousin of his friend.
Online, it is spread as a copypasta on sites like Reddit and Imgur.
The story was posted to the /b/ board of 4chan on January 19th, 2013, by an anonymous user.
The extensive story builds up, as the poster explains his deep ties with a girl, he had a crush on since kindergarten and with whom they were in a budding romantic relationship.
As things unfold, the girl is seduced by the older cousin of OP’s friend, all the way to having sex.
At the end of the story, the girl is sitting in the lap of the cousin, kissing him, when OP enters the room.
She then says to the other friends present “don’t worry, he knows”.
The story gained a lot of attention on 4chan, already as it was being written.
It would later be referenced by the phrase “don’t worry, he knows” in the title of similar stories and anecdotes.
In 2015, the copypasta would be reposted on the r/greentext sub of Reddit as well as Imgur.