When someone behaves in a surreptitious way, they're being secretive. They're doing something that they don't want to be seen doing.

While surreptitious means secret, it has the added sense of "sneaky" or "hidden." During the Jewish Passover meal of Seder, an adult will surreptitiously place a piece of matzoh somewhere in the house for the children to hunt for later in the meal. You'll see surreptitious applied mostly to actions, rather than to things or ideas. We do things surreptitiously. The members of the secret society hold surreptitious meetings because, well, they're a secret society. I was very surreptitious in how I organized the surprise party: she never knew!

Definitions of surreptitious

adj marked by quiet and caution and secrecy; taking pains to avoid being observed

“a surreptitious glance at his watch”
furtive, sneak, sneaky, stealthy
hidden on any grounds for any motive

adj conducted with or marked by hidden aims or methods

surreptitious mobilization of troops”
clandestine, cloak-and-dagger, hole-and-corner, hugger-mugger, hush-hush, secret, undercover, underground
secret or hidden; not openly practiced or engaged in or shown or avowed

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