a Middle English merger of Old English line "cable, rope; series, row, row of letters; rule, direction," and Old French ligne "guideline, cord, string; lineage, descent;" both from Latin linea "linen thread, string, line," from phrase linea restis "linen cord," from fem. of lineus (adj.) "of linen," from linum "linen" (see linen).
Oldest sense is "rope, cord, string;" extended late 14c. to "a thread-like mark" (from sense "cord used by builders for making things level," mid-14c.), also "track, course, direction." Sense of "things or people arranged in a straight line" is from 1550s. That of "cord bearing hooks used in fishing" is from c.1300. Meaning "one's occupation, branch of business" is from 1630s, probably from misunderstood KJV translation of 2 Cor. x:16, "And not to boast in another mans line of things made ready to our hand," where line translates Greek kanon, literally "measuring rod." Meaning "class of goods in stock" is from 1834. Meaning "telegraph wire" is from 1847 (later "telephone wire").
Meaning "policy or set of policies of a political faction" is 1892, American English, from notion of a procession of followers; this is the sense in party line. In British army, the Line (1802) is the regular, numbered troops, as distinguished from guards and auxiliaries. In the Navy (1704, e.g. ship of the line) it refers to the battle line. Lines "words of an actor's part" is from 1882. Lines of communication were originally transverse trenches in siegeworks.