adjective
surreptitious
sur-up-TISH-us/ˌsɜːrəpˈtɪʃəs/
Done secretly to avoid notice or disapproval.
The person takes a surreptitious cookie while checking whether the baker notices.

Etymology
“From under” contributes the idea of secrecy to “snatch,” producing a verb meaning “steal” or “seize secretly.” The adjective carries that furtive quality into actions done covertly or without legitimate authority.
In context
When he sold a work for an enormous price to one bookseller, even to Kramer, whom he really respected, he took care that a surreptitious edition should appear in Holland
More examples
- He slips the dog a surreptitious snack under the table.
- The students exchange surreptitious notes while the teacher writes.
- The investigation uncovers surreptitious payments to a public official.
Forms
surreptitiously adverb — Secretly, to avoid being noticed or disapproved of.
Similar words
- furtive
- Often suggests nervous behavior or fear of being caught.
- clandestine
- Often describes organized activities kept secret because they are forbidden.
- stealthy
- Emphasizes quiet, careful movement intended to escape detection.