Cut to the quick, meaning to offend deeply.
The saying is a metaphor, the idea that someone has cut so deeply, they are cutting at life itself, and here’s why. ‘Quick’ is actually from the old English ‘cwic’ which meant ‘alive’. ‘Cwic’ came to English with the Anglo-Saxons and was a variant of the Proto-Germanic ‘kwikwaz’ which meant ‘alive’ too. The reason it’s used to refer to the skin around the nails is to distinguish the alive part from the dead part (the nail).