meaning - Is 「うそおっしゃい」 to be taken literally here?

In the video game Ripening Tingle's Balloon Trip of Love, the protagonist Tingle meets a fortune teller who demonstrates her powers to him. She asks him, "You come from a far away town, right?" If Tingle answers, "No", she responds:

うそ おっしゃい ! おばさん わかってるの。
あなた まよっちゃったのよ ・・・

Now, from what I understand, うそおっしゃい means, "You're lying". However, since she continues with "I understand, you're lost...", it doesn't sound like she really disbelieves.

In this context, is うそおっしゃい to be taken literally, or is it a set phrase expressing astonishment like, "You must be kidding" ?

Answer

First, 「おっしゃい」 is the imperative form of the verb 「[仰]{おっしゃ}る」, which is the honorific form of 「[言]{い}う」.

「うそおっしゃい。」 means the exact opposite of what it means literally. Its literal meaning is "Tell a lie!", but that is clearly not something a person would say under normal circumstances, is it?.

Thus, 「うそおっしゃい。」 always means

"Don't lie (to me)!"

A more common form is:

「うそつけ！」

which also literally means "Tell a lie!", actually means "Don't lie!" 100% of the time.

The nuance of these phrases is "Lie all you want; I can see through you!", "Lie if you want but it won't work!", etc. Thus, you are, in essence, saying "C'mon, don't lie to me!" These expressions are loosely called 「[反語表現]{はんごひょうげん}」.

(Nothing to do with the question, really, but 「おばさん」 in this context means "I". The speaker is using it like a first-person pronoun in talking to a younger person. This is very common in Japanese.)