Loophole: Is made up of ‘Loop’, which actually originates in the Latin ‘loupa’, later ‘loupe’ in middle English, which meant a small window. And ‘hole’, which comes from the proto-Indo-European (PIE) word ‘kel’, which later became ‘hul’ in early German and ‘hohl’ in modern German.
Today loophole is used to mean an almost unnoticeable hole in a rule or a procedure. It gets its roots in the small windows in castles and battlements from which archers would shoot. The idea was that these small holes are analogous to the small gap which one finds in the rule.