"REEEEEEE" is an onomatopoeic expression of intense rage or frustration typically associated with the Angry Pepe character and used by those who frequent the /r9k/ board on 4chan. The expression is often associated with the Autistic Screeching meme, however it is intended to represent the unique croak produced by several species of frogs when agitated. Additionally, the expression has been combined with the word repost to call out duplicate posts on Reddit, Imgur and other sites.
Certain frog species are known to produce a shrill scream noise to ward of predators when threatened. On October 25th, 2009, the earliest known video of this behavior was uploaded by YouTuber ppparaone, which gained over 2.5 million views and 4,600 comments in the next six years (shown below). This video would later one become even more popular with a screenshot of a comment, and spread to humor content aggregator clickbait websites such as Buzzfeed  or Smosh .
The earliest known archived use of the onomatopoeic version of the scream was submitted to 4chan's /r9k/ board on December 27th, 2014, featuring an image of Pepe the Frog accompanied by the message "REEEEEEEEEEEEEEEEEEEEEEEEEEEEE" (shown below).
On January 23rd, 2015, a series of posts with Angry Pepe images featuring the cries of frustration appeared on /r9k/. On January 31st, a thread was submitted containing a picture of Pepe vomiting along with the expression, to which other users replied with audio clips of themselves making the screaming noise. Over the following weeks, additional posts were submitted featuring the onomatopoeic expression.
On February 21st, a Redditor submitted the question "Why do people on 4chan's r9k say 'REEEEEE' all of a sudden?" to the /r/OutOfTheLoop subreddit, to which Redditor Dragon___ replied that the sound was similar to shrieks made by autistic people (shown below). On March 19th, Redditor Taaqif submitted another question about "Reeeeeee" comments to /r/OutOfTheLoop.
On June 4th, 2017, Spirit Realm Games released a gameplay trailer for their mobile game Pepe Scream, a Flappy Bird-style game in which players control a depiction of Pepe the Frog by yelling "REEEEE!" into their microphone (shown below). The game was subsequently banned from Apple's App Store for containing images of Pepe the Frog, which Apple deemed "objectionable content."
On June 9th, Motherboard published an interview with the developers of the game, who revealed what inspired its creation:
“Why did we make a game about Pepe? We saw some memes featuring Pepe screaming ‘REEEEE’ and something just ‘clicked.’ We thought it was a silly and funny idea to just show off to our friends, nothing more, so we built it just for fun.”
"Fucking Normies! REEEEEEE!"
"Fucking Normies! REEEEEEE!" is an expression indicating rage against 'normies',, which is associated with the catchphrase "Fucking normies! Get out of my board!" used in audio recordings submitted to /r9k/. On February 1st, 2015, YouTuber GoldenSodaXbox uploaded one of the recordings (shown below, left). On February 24th, YouTuber The Dank Memester uploaded a video titled "NORMIES GET OUT," featuring several of the audio clips (shown below, right).
The audio clip inspired other parodies with various /r9k/ audio clips (shown below).
 Archived.moe – ALL RIGHT NORMIES! THE DAY OF YOUR DEMISE HAS COME