The scene: A young man named Akira is on a couch, being spoken to by Mina, a small blond with hair in some sort of pigtails. Akira only recently recovered from some significant amnesia.
She asks him, “So, you remember our promise, right?” He replies, “Mm? What promise?” Though the details are not exchanged at this juncture, in many Asian dramas, such a promise would be, for instance, I will always protect you, I’ll marry you when we’re older, that sort of thing.
Mina, deeply embarrassed, concedes that his lack of recall cannot be surprising, as he only just regained his memory about other things. and shifts away from the subject and prepares to leave. At this point, Akira sticks out his tongue to her.
“Uso.” (Over-pronounced, it sounds like us-so.)
A miffed Mina promptly begins beating on Akira’s head, drawing ows and yelps from the latter.
“Uso” is a word that, like many, has different possible readings in practice. The plainest reading from a dictionary will be, “lie.” In other words, a lie. The next reading, “a falsehood,” is simply a different version of a lie, but said in a slightly more diplomatic way.
For “uso” to be a complete sentence, there are two hidden, unstated components: the subject (Akira’s memory loss) and an existence verb (to be or not to be, that is the question). A complete, ridiculously elaborate version (with alternates) would read: “(What I said/ how I acted just now) (was) a lie/ false/ untrue.”
However, language is not about the equivalent coughed up by a dictionary; it is not even about what we might call the literal meaning. While “uso” certainly not true, an untruth, it is spoken here like an interjection. As such, we must look at not just what it says, but what it means.
In this context, what Akira could come off more like, “Just kidding.” However, typically “joudan da” would be used in that role.
Akira’s words could come off more like, “Fooled ya’.”
Perhaps we might even read it as, “Sucker.”
Thus, Mina immediately – and correctly – begins whacking Akira’s head, looking quite annoyed in the process.
(Source: “Dance in the Vampire Bund, episode 3)