loophole (n.)

also loop-hole, mid-15c., from hole (n.). + Middle English loupe "narrow window, slit-opening in a wall" for protection of archers while shooting, or for light and ventilation (c. 1300), which, along with Medieval Latin loupa, lobia probably is a specialized word from a continental Germanic source, such as Middle Dutch lupen "to watch, peer." Figurative sense of "outlet, means of escape" is from 1660s.