Adverb
in a furtive manner
Source: WordNetthe soldiers were furtively crawling through the night Source: Internet
After an interlude in the woods, where many have gone to hide, Max meets on the sly with Uncle Freddy, a closeted gay relative, who has a ticket to Amsterdam for him. Source: Internet
Ev began sleeping with Ruth on the sly, and he told Fanny that if only he were free, he'd be glad to marry her daughter. Source: Internet
His only hope: to win a 2-million dollar golf tournament (held on the sly at the local links) and pay Risa off. Source: Internet
I don’t know where my eight pounds landed, but the metamorphosis our bodies make on the sly amazes me. Source: Internet
That time ancient Etruscans stored their souls in a lake and your father was harvesting organs on the sly. Source: Internet