The adjective fly, also found as flee in Scotland, and as vly in Somerset, "sly, cunning, wide-awake, knowing, smart, sharp", as defined by the English Dialect Dictionary, is still found in general dialect and slang use. It first became prominent in the 18th century, where we find, "A the malt-man is right cunning but I can be as flee," in Ramsay's Tea Table Miscellany of 1724. In Mayhew's London Labour and the London Poor (1851) we find, "He was convinced that I was fly," and again he refers to "a public-house whose landlord is fly to its meaning." Dickens used the word in Bleak House: "'I am fly,' says Jo, 'but stow hooking it.'"

The word was used by the Scottish writer Blackmore in Kit (1890): "He was much too fly for any game of that sort." The word was recorded in both Australia and America in the 19th century. "He was pretty fly and never
threw away a chance as long as he was sober," wrote the Australian Boldrewood in Robbery in 1888; the prestigious Kansas University Quarterly described someone in 1892 as "a fly young man".

A witty use of the adjective is found in a 19th-century broadside ballad: "And is it not uncommon fly/ On them as rules the nation,/
To make us end with Botany/ Our public education?"

The word is still used in Yorkshire and in Somerset of a fast lady, light in character, "weak in the carnalities", in Seán O'Faolain's phrase. A fly-by-night is a Cornish phrase used of "a girl not only giddy but of unchaste habits," according to the English Dialect Dictionary. A fly-by-sky is another name for a similar type of girl; in Yorkshire and Lincolnshire it was applied to a gaudily or provocatively dressed woman. From Lincolnshire the EDD has: "She was ribbins an floonces fra head to foot when she run'd away wi' another woman's husband. I says it's abargans what end coms fo'st to a fly-be-sky like that."

A fly-by-sky is also applied to a fussy, forgetful person, young or old, in the north of England, and also to "one who fusses upon errands with but a trivial motive. A child who is running off on a message before the mother has finished imparting the instructions is checked by her calling out, "No, fly-by-sky, come back and listen to what I'm telling you."

I heard both fly-by-night and fly-by-sky in the Co Wexford of my long-lost youth. Gone now, I'm told, like many another old word and phrase.