You've probably heard of this word if you watch anime, and maybe heard some people say it. Since you're here, I suppose you want to know what 'desu' means. Let's get on with it! (And if not, enjoy the drawing with lots of cats.) - WARNING - LONG TEXT - You may or may not have heard of this common myth: "Desu is not a word in Japanese." That is INCORRECT. Yes, it can be said it is not a word but it depends on it's context. The word desu is a word in Japanese, and is a word that has its own specific meaning even when used outside a sentence.
The most commonly translated definition of the word 'desu' into English is a verb, 'it is' or 'to be'. But the fact that this myth of 'desu' is not a word is going around, is because a lot of people think the word 'desu' is just used to end a sentence; and they're not wrong.
The biggest reason why it's really difficult to define 'desu' to English is because the meaning changes depending on the suffix it follows. Let's use this sentence as an example: "ルーシーは馬鹿です。" (Rūshī wa bakadesu.) (You don't pronounce the 'u' in 'desu'.) "Lucy is an idiot." Now what obviously makes sense is that thinking that the word 'desu' in this situation would be directly linked to 'is an'. "ルーシーは馬鹿です。" "Lucy is an idiot." Although this isn't a direct translation, it's a correlated translation as its meaning is very similar or follows the same traits as 'is an'.
Now if we grow out the sentence and put 'ka' at the end of it, "ルーシーは馬鹿ですか？" , this then translates to a question: "Is Lucy an idiot?". And not only does the statement turn into the question, the syntax of the English translation changes around, but the word 'is' and 'an' is still in the word.
However, the biggest twist here is that we can say both of the statement and this question without the word 'desu' would still make sense. If you replace the 'desu' in the statement and question to the word 'da' (*In the question, you only get rid of 'desu' instead of replacing it with 'da'), the translation still stays the same: "ルーシーは馬鹿だ。" "Lucy is an idiot." "ルーシーは馬鹿か？" "Is Lucy an idiot?" So, what's going on here? Why is it that we got rid of the word 'desu', in both the statement and question, and yet it still has the same meaning? Well, this is where the myth of "'Desu' is not a word in Japanese." comes into play. The other thing that really defines the word 'desu', is that it is used to end a sentence in formal Japanese. If you don't know, Japanese has two main ways of speech: formal and informal. In formal Japanese 'desu' is used to end any kind of sentence or statement, and in informal Japanese there are other (multiple) ways to end a sentence other than 'desu'. That hints where 'da' comes from to end a sentence and still making sense even without the word 'desu' being in there.
But a lot of people forget that you can actually use the word 'desu' without anything before, since it still holds the single definition of 'it is' or 'to be'. Lets go back to the question: "ルーシーは馬鹿ですか？" "Is Lucy an idiot?" Now, in Japanese there are a number of ways that I can confirm this question. I can say "ルーシーは馬鹿です。" ("Lucy is an idiot."), in other words a declarative statement to answer the question: "Yes, Lucy is an idiot.". But in Japanese context is really important, and so in this situation I can easily answer with just saying "馬鹿です。" ("Bakadesu."); I can even go as far as to just say "です。". Just like I said, context is very important. If I just said "です。" without any kind of question or statement before the situation, then "です。" would make no sense; it would hold no meaning. But since I'm saying "です。" after this particular question, "Is Lucy an idiot?". By answering with just "です。", I'm implying that I have turned that question into just a statement, and I am declaring that question as a statement. Are you confused yet? I think you are.
Now look, all of you anime viewers and all going around "です, です, です, です。" to end all your sentences (I mean check out the urban dictionary definition of you want a good laugh). But in real world Japanese, in conversational Japanese, the word 'desu' is very confusing and very important because it has a definition and is a word, but doesn't have a definition and isn't a word at the same time. Welcome to Japanese, a world of confusion. But if you would to take anything out of this Japanese 101, just remember that the word 'desu' is very ambiguous and obscure word, and is also not a word at the same time. In Japanese, context is everything and it can turn a word into a word, and it can turn a word into just something into a period of a full stop. Because that's essentially what 'desu' is, and that's essentially what English speaking people have really hard troubles to understand when learning Japanese, is the way they use context in their sentences and their sentence structures is very different from English.
So, if you thought Japanese couldn't get any harder: think again. I hoped you, the reader, thought this was useful and learned something from it :) Please comment what you would like to know about next, in the next Japanese 101! ^.^ Resource(s): The Anime Man