Slip of the mouth instead of a slip of the tongue?

Question

This sentence is in my JLPT textbook:

つい舌{した}がすべってしまい、うそがばれてしまった。

I thought that it meant something like, "with an unintentional slip of the tongue, my lie was exposed." However, the sentence is marked as being an incorrect usage of すべる, and it doesn't explain why.

I know that one can say つい口{くち}がすべる, which in English I would equate with "a slip of the tongue," to accidentally say something one didn't intend to say.

In Japanese, is it only mouths that slip, not tongues?

(A helpful answer would not only confirm this with a yes or no, but ideally offer a little insight on how 口{くち} and 舌{した} differ in metaphorical terms.)

Answer 1

Probably the reason the book highlighted this is that the set phrase in Japanese, 口がすべる, is easily confused with the literal translation of a very similar set phrase in English, 舌がすべる。

I don't think this points to an overall difference in the metaphorical uses of 口・舌 (see also the word 舌禍{ぜっか}), just one of many cases where caution is needed. I don't know the book so I don't know if this is something they regularly do - it would have been better if they'd added something along the lines of "this is not the collocation you are looking for, try 口がすべる instead".

Answer 2

Yes - it would seem mouths slip not tongues.

The book is testing your recognition of the expression 口が滑る, trying to throw you off by inserting 舌. (Its probably a common technique for the other questions you are looking at)

The best insight I can suggest for this expression is to look up 口 and 舌 in the dictionary to see how they are used. 口 tends to be used more for words/talking eg:

性格はいいけど口が悪い。

But it is not hard and fast. There are also expressions about speaking that do use tongue:

舌がもつれて何も言えなかった｜I was tongue-tied

よく舌が回るね｜〔批判的に〕You really do [go on / talk].

But the main point for this book is to remember uses of 滑る