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."
The meaning "consisting of one unit, individual, unaccompanied by others" is from late 14c., often merely emphatic. The 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.). Of single things from 1640s. From the broad sense of "that which is single" it was given various technical meanings from 16c.
The sports senses are attested from 1851 (cricket, "hit for which one run is scored") and 1858 (baseball, "one-base hit"). The meaning "one-dollar bill" is by 1936. The meaning "phonograph record with one song on each side" is from 1949. The sense of "unmarried swinger" is from 1964; singles bar, catering to the young and unmarried, is attested from 1969. An earlier modern word for "unmarried or unattached person" is singleton (1937).
"to separate from the herd" (originally in hunting, often with forth or out), "select individually from among a number," 1570s, from single (adj.). The baseball sense of "make a one-base hit" is from 1899 (from the noun meaning "one-base hit," which is attested from 1858). Related: Singled; singling.
mid-14c., singuler, "alone, apart; being a unit; special, unsurpassed," from Old French singuler "personal, particular; distinctive; singular in number" (12c., Modern French singulier) or directly from Latin singularis "single, solitary, one by one, one at a time; peculiar, remarkable," from singulus "one, one to each, individual, separate" (see single (adj.)).
In grammar, "relating to one person or thing," late 14c. The meaning "remarkably good, unusual, rare, separated from others (by excellence), uncommon" is from c. 1400 in English; this also was a frequent meaning of Latin singularis. The meaning "out of the usual course, somewhat strange" (shading toward "eccentric, peculiar") is by 1680s.