1595-1605; apparently of multiple orig.; in senses referring to a gliding or smooth transition, compare Low Germanslurren to shuffle, Dutchsleuren to trail, drag; in senses referring to a smirch or stain, compare Middle Dutchslore (Dutchsloor) sluttish woman
"deliberate slight," 1609, from dialectal slur "thin or fluid mud," from M.E. slore (1440), cognate with M.L.G. sluren, M.Du. sloren "to trail in mud." Related to E.Fris. sluren "to go about carelessly," Norw. slora "to be careless." The musical sense (1746) is from the notion of "sliding."