c. 1400, sailer, "one who sails," agent noun from sail (v.). The spelling with -o-, erroneous but now established, arose 16c., probably by influence of tailor, etc., and to distinguish the meaning "seaman, mariner" from "thing that sails."
It replaced much older seaman and mariner (q.q.v.), and its later appearance is perhaps to avoid confusion with common Middle English saillour, sailer "dancer, tumbler, acrobat" (mid-13c. as a surname), from Old French sailleor (from Latin salire "to leap"). Old English also had merefara "sailor."
Applied as an adjective from 1870s to clothing styles and items based on a tailor's view of a sailor's characteristic attire. Vulgar extended form sailorman is by 1761. Sailor's purse "egg pouch of a ray or shark" is so called by 1874; it is typically empty when found on shore.