expression, godan verb (-ru)
say repeatedly; talk until one is blue in the face
Idiomatic expression used when someone has said the same thing many times, often to the point of frustration. Literally 'one's mouth becomes sour'.
口が酸っぱくなるほど注意したのに、彼はまた同じミスをした。
I warned him until I was blue in the face, but he made the same mistake again.
母は
My mother told me to study so many times that her mouth went sour.