So embarrassing that your whole body reacts — the picture in the word is your toes bending up tight inside your shoes. You use it about something you watch or hear, not usually about how you yourself feel: a bad speech, a joke that no one laughs at, a singer who cannot sing, a parent dancing at a wedding. The key point is that the embarrassment is often not yours. You feel it for the other person, and that is worse. English has a whole family of words for this — cringeworthy, excruciating, painful (in this sense), and now simply 'cringe' among young people — and 'toe-curling' sits comfortably among them. It is informal but not rude, and adults use it freely in newspapers and reviews. It is more common in British English than American. You will see it before a noun most of the time: a toe-curling moment, toe-curling embarrassment, toe-curlingly bad. One more thing: 'toe-curling' can also describe pleasure so intense that your body reacts the same way, especially about sex — 'a toe-curling kiss'. The physical reaction is identical; only the cause changes. Context always makes it clear which one is meant, but knowing about the second use will save you from a misunderstanding one day.