Originally and literally the denizens of Saraka, a territory in Arabia (present-day Yemen); Stephanus of Byzantium s.v.; Barrington Atlas map 4 grid B3. However, in late-antique and early-medieval usage the application of the term progessively broadened out, first to include non-Arab people(s) of the region and ultimately to become a generalised synomym for Arabs/Muslims.
See also alpha 3278, alpha 4027, gamma 149, kappa 527, lambda 549, mu 721, omicron 346, rho 149.
On the history of this designation, see Wikipedia entry at web address 1.