Old English turf, tyrf "slab of soil and grass, sod," also "surface of grassland," from Proto-Germanic *turfa- (source also of Old Norse torf, Danish tørv, Old Frisian turf, Old High German zurba, German Torf), from PIE root *drebh- "to wind, compress" (source also of Sanskrit darbhah "tuft of grass").
Especially "the race course," hence the turf "the profession of racing horses" (1755). French tourbe "turf" is a Germanic loan-word. The Old English plural was identical with the singular, but in Middle English turves sometimes was used. Slang meaning "territory claimed by a gang" is attested from 1953 in Brooklyn, N.Y.; earlier it had a jive talk sense of "the street, the sidewalk" (1930s), which is attested in hobo use from 1899, and before that "the work and venue of a prostitute" (1860). Turf war is recorded from 1962.