Nan da yo

[Sai]

I've heard this expression a couple of times and am curious, what does it exactly mean?

[Max]

would be pretty hard to say what it exactly means, but something like "what is it!"

[Lloyd Wilson]

isnt it a plain form of nan desu ka? which is what max has said. what/ what is it

[Max]

it could be...
犬なんだよ... there's a sentence when it isn't...
I need context :)

[Lloyd Wilson]

oh oops:P u hear it alot in anime if that helps:P and it is usually by itself... when something is happening:)

hope this helps lol:)

[Max]

in that case, it has a nuance of "what do you want"... but means something more like "what is it!?" it's not very polite though.

[Max]

by the way, the plain form of "nan desu ka" would be "nani" (^_-)

[Shia]

Just to summarise (and clarify):
- Llody Wilson is technically correct: da is the plain form of desu. Yo in this case is like an exclamation mark to add emphasis
- Say if you're pointing or looking at SOMETHING and say "nan da yo", it kinda means as Max said "what is it!".
- But say SOMEONE approaches you and you say "nan da yo", it means as Max said "what do you want". For example: kondo ha nan da yo ("what is it THIS time" or "what do you want this time"). Otherwise you could have said it in a more polite manner as Lloyd Wilson pointed out "nan desu ka"

[Aya]

I don't think it's very polite. When I watch Japanese dramas and movies, they only use 'nan dayo' when they are angry with someone.