c. 1200, from Old Norse skaða "to hurt, harm, damage, injure," from Proto-Germanic *skathan- (source also of Old English sceaþian "to hurt, injure," Old Saxon skathon, Old Frisian skethia, Middle Dutch scaden, Dutch schaden, Old High German scadon, German schaden, Gothic scaþjan "to injure, damage"), from PIE *sket- "to injure." Only cognate outside Germanic seems to be in Greek a-skethes "unharmed, unscathed."
It survives mostly in its negative form, unscathed, and in figurative meaning "sear with invective or satire" (1852, usually as scathing) which developed from the sense of "scar, scorch" used by Milton in "Paradise Lost" i.613 (1667).