Mezzotint and atipple engraving, 14x10cm, fold across the plate, anonymous artist, 19th century, after The Oxford English Dictionary defines ‘sly boots’ as ‘a sly, cunning, or crafty person; one who does things on the sly,’ and notes that the phrase is usually applied in ‘mild or jocular use.’ Its not a phrase used in anger, in other words, but the sort of thing you say when you discover youve been mildly deceived (‘Oh, you sly boots. You snuck a seventeenth kitten into the house!’).
‘Sly boots’ is a very old phrase, defined (‘a seeming silly, but subtle Fellow’) in Nathan Baileys 1721 Dictionary of Canting and Thieving Slang, and probably a good deal older. ‘Sly,’ of course, means ‘cunning, clever or wily,’ and comes from an Old Norse word meaning ‘crafty.’ ‘Boots’ is the interesting bit, originally, in the 17th century, used as slang for a servant in a hotel who cleaned the guests boots. It was also used to mean the most junior officer of a regiment or member of a club, the one most likely to be stuck with menial chores (‘My chief resistance to discipline was at mess where I could not brook the duties of Boots..,’ 1806). ‘Boots,’ used as a synonym of ‘fellow,’ also found its way into various humorous and colloquial phrases of the period, such as ‘smooth boots’ (one who is adept at flattery and manipulation), ‘clumsy boots’ and ‘lazy boots.’