Other forms: cuts to the quick; cutting to the quick
Unkind words can cut to the quick, meaning they can really hurt, almost like a knife to your inner self. If someone says or does something that cuts to the quick, resist the urge to retaliate.
The adjective quick usually means "fast," but it used to also mean "alive" or "living." As a noun, the quick is the tender, sensitive flesh under your fingernails, because unlike the nail itself, this flesh is living. If you’ve ever accidentally had something sharp pushed under your fingernail, such as a splinter, you know how terribly it hurts! The idiom cut to the quick suggests this kind of sharp pain to the emotions.