Arthur's Fist is a reaction image featuring a screen capture of the protagonist Arthur from the titular children's television series holding a clinched fist, which is often accompanied by captions describing various infuriating or frustrating circumstances. The image is taken from the Arthur Punches D.W. scene, notable for inspiring a series of YouTube poop videos.
On September 6th, 1999, Episode 1 Season 4 of Arthur titled "Arthur's Big Hit" was broadcast, during which Arthur is shown clinching his fist before punching his little sister D.W. for breaking his Bell X-1 model airplane (shown below).
On July 27th, 2016, Twitter user @AlmostJT posted a screen captured image of Arthur's fist, describing the picture as "relatable" for displaying "so many emotions in one fist" (shown below). The original tweet has since been removed.
On July 28th, 2016, Redditor axedowg submitted the fist image with the caption ”when people say 'Harambe was just a gorilla'” to /r/blackpeopletwitter, where it gathered upwards of 4,800 votes (89% upvoted) and 130 comments within five days (shown below, left). Meanwhile, the Arthur__Hands Twitter feed was launched, featuring various examples of the Arthur fist meme (shown below, right).
That day, Paper Mag and The Daily Dot published articles highlighting notable examples of Arthur fist tweets. On July 30th, Redditor MGLLN posted a photoshop of Arthur's fist punching North Korea to /r/blackpeopletwitter (shown below, left). In 72 hours, the post received more than 6,300 votes (89% upvoted) and 140 comments. On August 1st, Redditor Aerolas submitted a picture of Spongebob Squarepants holding a clenched fist with the caption "Arthur memes are better than Spongebob memes" to /r/blackpeopletwitter (shown below, right). That day, The Verge published an article about trending image titled "The Arthur fist meme is the best new meme in a long line of Arthur memes."
LeBron James Usage
On November 7th, 2017, basketball star LeBron James posted a picture of Arthur's fist to his Instagram account with the caption "mood" (shown below). This led to speculation that James was frustrated with his team, as Arthur's Fist is typically used to symbolize frustration.
He responded to questions about the post by simply saying "I like Arthur." Shortly after the post speculation, James posted a compilation of photos pumping his fists in celebration along with Arthur's Fist, apparently indicating that he intended the fist to symbolize him being pumped up. This led SB Nation to speculate if James actually understood what memes meant.
John Legend Costume
On February 21st, 2017, Chrissy Tiegen posted an Arthur's Fist joke in response to people saying her husband, John Legend, looked like Arthur (shown below). Her tweet gained over 216,000 retweets (shown below).
On April 25th, 2018, Legend posted an ad he starred in for Google Duo which referenced the meme. The video gained over 19,000 retweets and 50,000 likes (shown below). The video was covered by Bustle.
 The Daily Dot – This meme of Arthurs clenched cartoon fist really says it all