on the sly
• in a secretive fashion.
• "she was drinking on the sly"
Similar:
in secret,
secretly,
furtively,
stealthily,
sneakily,
slyly,
surreptitiously,
covertly,
clandestinely,
on the quiet,
on the side,
behind someone's back,
under cover,
under the counter,
on the q.t.,
