Scathing means witheringly harsh. If you enter a singing contest and the judge says that your singing is like that of a toad with laryngitis, that is scathing criticism.
Scathing comes from an old Norse verb, to scathe, which means to injure by fire or lighting. Now we use it when someone’s critique is so harsh that it feels like you've been burned. If your best friend turns against you and advertises your faults to the world, and you feel like everyone is staring and talking about you, you are having a scathing experience.