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1771, "to draw a line under," from under + score (v.). The figurative sense of "to emphasize" is attested from 1891. Noun meaning "a line drawn below (something)" is recorded from 1901.
character _, ASCII 95. Common names: ITU-T: underline; underscore; underbar; under. Rare: score; backarrow; skid; INTERCAL: flatworm. See also left arrow. (1995-03-06)