c. 1300, maimen, "disable by wounding or mutilation, injure seriously, damage, destroy, castrate," from Old French mahaignier "to injure, wound, muitilate, cripple, disarm," a word of uncertain origin, 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- (1) "to cut."
In old law, "to deprive of the use of a limb, so as to render one less able to defend or attack in fighting." Related: Maimed; maiming. It also is used as a noun, "injury causing loss of a limb, mutilation" (late 14c.), in which it is a doublet of mayhem.