Surreptitious

adjective/sur-ep-tish-us/

Meaning

kept secret, especially because it would not be approved of.

In context
The surreptitious glances between the two hinted at an unspoken secret.

Remember it

SUR + REPT: Latin 'creep under' — surreptitious acts creep along secretly, hoping no one notices.

Use it well

Use for sneaky actions in narratives: 'a surreptitious glance/note' — stronger and more formal than 'sneaky'.