Context: Conrad Greene is talking on the phone about the painting whereabouts.
Oh, it's you! Where's my painting? I want my painting. In the mail? Well, that's original!
Sentence taken from "Entrapment (1999)" movie.
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The literal meaning of the phrase "That's original" would mean "That's unusual" / "That's unexpected".
Here, though, it's being used sarcastically, as this phrase very commonly is. The sarcasm is because "It's in the mail" is seen as a stereotypical excuse when someone hasn't mailed or shipped something as promised.
So the sarcastic response means basically "That's the excuse I expected you to give me".