scare someone witless
scare (one) witless
To shock or frighten one very severely. (Hyperbolically alludes to scaring one so badly that they lose their sanity.) Don't sneak up on me like that, you scared me out of my wits! That car accident seems to have scared Janet out of her wits—she's still shaken by it.
scare someone witlessINFORMAL
If something or someone scares you witless, they make you very frightened or worried. I saw that film when I was in Paris and it scared me witless. You read about all these things that can cause cancer and it scares you witless.