on the sly

adjective, sly·er, sly·est.

1. cunning or wily: sly as a fox.
2. stealthy, insidious, or secret.
3. playfully artful, mischievous, or roguish: sly humor.

Idioms

4. on the sly, secretly; furtively: a tryst on the sly.

adjective slyer, slyest, slier or sliest

1. crafty; artfula sly dodge
2. insidious; furtivea sly manner
3. playfully mischievous; roguishsly humour

noun

4. on the sly in a secretive manner

adj.c.1200, “skillful, clever, dexterous,” from Old Norse sloegr “cunning, crafty, sly,” from Proto-Germanic *slogis (cf. Low German slu “cunning, sly,” German schlau), probably from base *slak- “to strike, hit” (see slay (v.)), with an original notion of “able to hit.” Cf. German verschlagen “cunning, crafty, sly,” schlagfertig “quick-witted,” literally “strike-ready,” from schlagen “to strike.” A non-pejorative use of the word lingered in northern English dialect until 20c. On the sly “in secret” is recorded from 1812. Sly-boots “a seeming Silly, but subtil Fellow” is in the 1700 “Dictionary of the Canting Crew.” Furtively, secretly, as in She’s always eating cookies on the sly. The adjective sly, which means “cunning” or “crafty,” is here used as a noun. [c. 1800] see on the sly.