line (n.)youdaoicibaDictYouDict
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, as in 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.
party (n.)youdaoicibaDictYouDict
late 13c., "part, portion, side," from Old French partie "side, part; portion, share; separation, division" (12c.), literally "that which is divided," noun use of fem. past participle of partir "to divide" (see part (v.)). Political sense of "side in a contest or dispute" evolved by 1300; meaning "a person" is from mid-15c. Sense of "gathering for social pleasure" is first found 1716, from general sense of persons gathered together (originally for some specific purpose, such as dinner party, hunting party). Phrase the party is over is from 1937; party line is first recorded 1834 in the sense of "policy adopted by a political party," 1893 in the sense of "telephone line shared by two or more subscribers." Party pooper is from 1951, American English.