sly

Concise Oxford English Dictionary © 2008 Oxford University Press:
sly/slʌɪ/

▶adjective (slyer, slyest)

• 1 having a cunning and deceitful nature.

• 2 (of a remark, glance, or expression) insinuating.

• 3 (of an action) surreptitious.

– phrases
on the sly in a surreptitious fashion.

– derivatives
slyly (also slily) adverb,
slyness noun.

– origin ME (also in the sense ‘dexterous’): from ON slœgr ‘cunning’.