early 14c., "unmarried," from Old French sengle, sangle "alone, unaccompanied; simple, unadorned," from Latin singulus "one, one to each, individual, separate" (usually in plural singuli "one by one"), from PIE *semgolo‑, suffixed (diminutive?) form of root *sem- (1) "one; as one, together with." Meaning "consisting of one unit, individual, unaccompanied by others" is from late 14c. Meaning "undivided" is from 1580s. Single-parent (adj.) is attested from 1966.
c. 1400, "unmarried person," mid-15c., "a person alone, an individual," from single (adj.). Given various technical meanings from 16c. Sports sense is attested from 1851 (cricket), 1858 (baseball). Of single things from 1640s. Meaning "one-dollar bill" is from 1936. Meaning "phonograph record with one song on each side" is from 1949. Meaning "unmarried swinger" is from 1964; singles bar attested from 1969. An earlier modern word for "unmarried or unattached person" is singleton (1937).
"to separate from the herd" (originally in deer-hunting, often with forth or out), 1570s, from single (adj.). Baseball sense of "to make a one-base hit" is from 1899 (from the noun meaning "one-base hit," attested from 1858). Related: Singled; singling.