slur (n.) Look up slur at Dictionary.com
"deliberate slight," c.1600, from dialectal slur "thin or fluid mud," from Middle English slore (mid-15c.), cognate with Middle Low German sluren, Middle Dutch sloren "to trail in mud." Related to East Frisian sluren "to go about carelessly," Norwegian slora "to be careless." The musical sense (1746) is from the notion of "sliding."