Desu

[Couch Potatoe]

japanese for it is, often said at the end of sentences to seem cute or unwitting.

[Luis Fok]

A Japanese word commonly used to indicate the speaker is a retarded wannabe Japanese anime-whore

As in:

[Little_Girl]

Or です in Hiragana. Desu is the polite form of the copula in Japanese linguistics. This translates out to to be or is. Its usually used at the end of a sentence as the predicate, or is used to make a sentence sound more polite. The plain/casual form for this is da, or だ. Sentences that use the copula usually take on the form of A is B type sentences, although it can take on other moods depending on the grammatical particles that accompany it.
Unfortunately, the copula has been bastardized and misused too often by people who dont know anything about Japanese. If you want to sound like a complete fucktard, use desu at the end of your sentences while speaking in English, or use it excessively at the end of every sentence, even when the sentence doesnt require it to be used.

[Dave Paprocki]

The one word that every real Japanese person will go insane if you keep on saying it to them, mainly standing for it is to go at the end of a sentence became a big [meme] of annoying [weaboo] or [otaku]. If youre wondering why it goes at the end of the sentence its because in Japanese sentences are formed differently.

It is said in a [motivational Poster] that if you keep on typing Desu, you will turn Japanese, people believed that, and thats now what you will constantly hear from people who want to prove theyre Japanese, also see [kawaii] for another annoying weaboo term.

[Jason Yan]

A word that bears different meanings when used by those familiar with the Japanese language from those who speak not a lick of Japanese yet want to seem like they know what theyre talking about, AKA your garden-variety weeaboo.
When used by an actual Japanese speaker, it roughly translates to it is and is an integral part of Japanese grammar. When used by your average weeaboo, it roughly translates to I am a stupid American faggot who has no idea what the fuck theyre talking about.

[Jessica]

mean ? in japanese

[Justin Bieber]

It a meme on 4-chan to let newfags release the immense faggity inside them. It can also be used by trolls and other such assholes for the same reason. It is often spammed in ridiculous amount and pisses most people off. It has NOTHING to do with pwning or awesomeness.

[Jason Yan]

Desu is the sound made by [pwning]. Conversely, by repeating desu, you will have pwned your opponent(s), even if you already lost according to [Godwins Law].