late 14c., "a barrier, a fence," from Old French closure "enclosure; that which encloses, fastening, hedge, wall, fence," also closture "barrier, division; enclosure, hedge, fence, wall" (12c., Modern French clôture), from Late Latin clausura "lock, fortress, a closing" (source of Italian chiusura), from past participle stem of Latin claudere "to close" (see close (v.)).
Sense of "act of closing, a bringing to a close" is from early 15c. In legislation, especially "closing or stopping of debate" (compare cloture). Sense of "tendency to create ordered and satisfying wholes" is 1924, from Gestalt psychology.
1800, "one who is on the outside" of a boundary, barrier, etc., from outside; figurative sense of "a person isolated from conventional society" is first recorded 1907. The sense of "a race horse not included among the favorites" is from 1836; hence outside chance (1909).