A meme that started from 4chan and comes from Suiseiseki of Rozen Maiden, and her tendency of saying "desu" after everything. The starter of the meme is when /a/ spammed /b/ with it, and they adopted it as a meme. It is usually used as spam.
The tag should only be used if "desu" or "です" appears in the image (either by itself or if there is an emphasis on the word), or in rare cases where the character Suiseiseki is used as a replacement for the term (post #266173).
More recently associated with Kongou of Kantai Collection as well, who also tends to use this word a lot.
