on sly (adverb) covertly: clandestine, underhandedly
Sinonime: bashful, clammed up,
close, close-mouthed, dried up, dummied up,
hesitant, mum,
reserved,
restrained,
shy, silent, taciturn, tight-lipped, uncommunicative, unforthcoming, unspeaking,
uptight*
Antonime: communicative,
forward,
unrestrained