exactly what does this means (slang?)

I hear this often, both in Tokyo and on television dramas and I assume it's slang or a really informal way of saying 'thank you', but I'm really not sure.

it sounds like 'wa ari'.. just that.

Rick,

If you can give an example of a full sentence/exchange in which you heard it, it might be easier to give a precise answer.
"Ari" (from "arieru") is often used by younger people to mean "OK", "acceptable", "doable", or "feasible", so I would assume that this is what it is if you are hearing "XXXX wa ari".

I think it could be "悪い"
(warui, waarii, warii, & other pronouncitation) the meaning is "thank you" in formal way

If that phrase comes only from guys (I hope so), and it sounds like "warii," it's a VERY VERY colloquial way of saying "THank you" - the original form "warui" in this context means "I am troubling you" = "I owe you" = Thank you.

Yeah, i do hear it only from guys. Had a feeling it was 'thank you'. thanks :)