stopgap (n.) also stop-gap, "that which fills a hiatus, an expedient in an emergency," 1680s, from stop (v.) + gap (n.); the notion probably being of something that plugs a leak, but it may be in part from gap (n.) in a specific military sense "opening or breach in defenses by which attack may be made" (1540s).

According to dictionary.com, stopgap is an adjectival use of the verb phrase "stop a gap" (meaning, essentially, plug a hole) and emerged around 1525-1535. Merriam-Webster, though, has the first known use as 1684.