maim: [13] Maim and mayhem [15] are ultimately the same word. Both go back to a Vulgar Latin verb *mahagnāre ‘wound’, whose origins are unknown. This passed into Old French as mahaignier (whose probable Anglo-Norman derivative *mahangler was the source of English mangle ‘mutilate’ [14]). Mahaignier became mayner, and passed into Middle English as mayn.
But it also had a noun derivative, mahaing or main, which in due course became mayhem. This seems to have been borrowed into English twice. First, in the 14th century, as maheym or maim ‘severe injury’; this has now died out, but has left its mark on the verb, which it has changed from mayn to maim. And second, in the 15th century, via Anglo-Norman, as mayhem. => mayhem
maim (v.)
c. 1300, maimen, from Old French mahaignier "injure, wound, muitilate, cripple, disarm," possibly from Vulgar Latin *mahanare (source also of Provençal mayanhar, Italian magagnare), of unknown origin; or possibly from a Germanic source, from Proto-Germanic *mait- (source of Old Norse meiða "to hurt," related to mad (adj.)), or from PIE root *mai- "to cut." Related: Maimed; maiming.