"The ass was fat" is a catchphrase used by a person who notices someone with a rear end which they consider attractive. The phrase spawned an image macro series, "Arthur Sees A Fat Ass", which features the title character of both the book series and American children's television show 'Arthur', peeking into a room and staring wide-eyed at what is assumed to be a person with a large or round butt. The text on the image usually shows Arthur describing one of the person's flaws (usually a physical one), however he then justifies his ogling of the person by saying "but the ass was fat."
"The ass was fat" was possibly the first Arthur meme to have appeared.
On February 7, 2012, an anonymous 4chan poster started a thread in which he requested pornographic images of the cartoon characters Arthur Read and Sue Ellen Armstrong, from the American children's animated series 'Arthur'. He went on to describe Sue Ellen, and another character from the show named Molly, in a very graphic and sexual manner. Most notably when giving these descriptions, he repeatedly used the phrase "the ass was fat". The following image shows the first post of the thread, along with several replies.
The post was met with disgust and ridicule from posters who believed that his request was legitimate, and laughter from other posters who believed that the thread was made as a joke. The phrase 'the ass was fat' was then repeated by other posters, and an image macro series was created in the same thread. The thread was also archived.
Several variations of the image macro can be found on Quickmeme. Also, on February 8, 2012, several Reddit users began submitting screencaps of the 4chan thread. One Reddit user by the name 'Zanhana' submitted a link to the screencap on the 'WTF' subreddit; it received 659 upvotes and an overall score of 399.