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 they're 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 they're talking about."
"Prussia is so kawaii, desu!"
"Shut up, you fucking Wapanese faggot."