Sly means cunning and keeping its purpose hidden. A sly remark, a sly look, sly as a fox.
It is always a criticism when applied to a person, and it implies concealment rather than mere cleverness. A sly grin or a sly look is one shared secretly. On the sly means secretly and usually improperly: doing it on the sly. Note slyly; and that sly, cunning and crafty are three words for nearly the same thing, all of which began as compliments about skill and became complaints about honesty.
1adjectiveslyCunning and keeping its purpose hidden.
“The fox is often shown as a sly animal in children's stories.”
synonyms: cunning, crafty
sly is a quality of hidden cunning.
Old Norse slaegr meant clever or skilful, from a root meaning to strike — somebody handy with a hammer. English kept the cleverness and added the suspicion, which is the same journey crafty made.
Proto-Indo-European
*slak- — to hit, throw
Proto-Germanic
*slōgiz — lively, agile, cunning, sly, striking
Old Norse
slǿgr — sly, cunning
Middle English
sly
English, today
sly
From Middle English sly, sley, sleigh, sleiȝ, from Old Norse slǿgr (“sly, cunning”) literally “capable of hitting or striking”, from Proto-Germanic slōgiz (“lively, agile, cunning, sly, striking”), from Proto-Indo-European slak- (“to hit, throw”). Cognate with Icelandic slægur (“crafty, sly”), Norwegian Nynorsk sløg (“sly”). Related to sleight, slay. In all likelihood, however, unrelated with stq slau (“sly, crafty”), Dutch sluw (“sly, cunning”), Low German slu (“sly, cunning”), German schlau (“clever, crafty”). a doublet of sleight and slöjd
Etymology from Wiktionary, CC BY-SA
Practised in A Song of Ice and Fire