Is it farther or further?
1794 in literal sense, present participle adjective from scathe (v.). Of words, speech, etc., from 1852. Related: Scathingly.
c.1200, from Old Norse skaða "to hurt, harm, damage, injure," from Proto-Germanic *skath- (cf. 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 root *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).