Origin
Definition
The Japanese word desu is a copula, a word that links the subject of a sentence with the verb. A close English approximation of this would be the words “this (that, he, she, it, etc) is” unless it’s coupled with an adjective or a verb. Regardless of how it is used, its main function is to end sentences intended to be a statement. Due to the grammatical structure in Japanese, desu and verb tenses are used at the end of a sentence or phrase. (ex. “This is Sushi” would be translated into “Kore wa sushi desu.” and “I like Sushi” would be “Sushi ga suki desu“)
Other Japanese copulas are da, de, wa, na, and more. The form desu is essentially the same as da except that it is used in a casual, but still polite or proper context.
Pronunciation
In standard Tokyo dialect, the u in “desu” is not pronounced, as if it were spelled “des.” The character of Suiseiseki pronounces the u, which is common in women in Northern Honshu and Hokkaido.
4chan
The earliest memetic usage of the word desu as a method of spam attack began in 2006 in a retaliatory forum raid on 4chan’s random imageboard, /b/. The raid was started by the anime board, /a/, after users from /b/ staged a raid of their own. Due to the tourettes-like nature of the word’s primary usage, it was quickly adopted by /b/ users. One of the earliest available examples comes from April 12th, 2006 (shown below).
Image macros using the word desu or images of Suiseiseki are used commonly on 4chan by fans of anime (sometimes referred to by the derogatory term weeaboo ), and by those mocking them.
Source: https://knowyourmeme.com/memes/desu