From Longman Dictionary of Contemporary Englishsly smile/glance/wink etcsly smile/glance/wink etcSECRETa smile, look etc that shows you know something secret He leaned forward with a sly smile. → sly
Examples from the Corpus
sly smile/glance/wink etc• All those gloating women looking down into his grave, whispering their sisterly secrets behind their sly smiles.• He was relieved when he saw Earnhardt's trademark sly smile.• Magrat took her seat and, to Agnes's amazement, gave her a sly wink.• Mitchell was shocked to see the man signaling him with sly winks.• Wickham saw Marshall's sly smile and understood what he was thinking.• Herbie's face had also lit in a sly smile as Mary had passed.• By his sly smile, I think this is the one he intended. 