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.