Fly

As in a 'fly person' or a person who is 'a bit fly' is an adjective that describes a knowing or artful person. According to the OED, it is slang that dates from 1811, and from 1844 its meaning was extended to describe a nimble or skilful person. The OED adds that etymological efforts to link it to the insect or the act of flying are very dubious. A fly is also the flap at the entrance of a tent, and this usage dates from 1810. A fly also describes the flap on a garment that covers the button holes, and this usage dates from 1844. See also 'on the fly'.