Surreptitious means done secretly so as not to be seen. A glance at a watch, a phone checked under a table.
The word is nearly always about small acts rather than large crimes, which is why a surreptitious look and a surreptitious sip are the natural uses. Nobody surreptitiously robs a bank. That smallness makes it faintly comic. The adverb surreptitiously is commoner than the adjective. Its neighbours are furtive, which suggests guilt, and covert, which is official and belongs to intelligence work.
1adjectivesurreptitiousDone quietly and secretly to avoid being noticed.
“He took a surreptitious look at his phone.”
Surreptitious creeps. Latin surrēpō meant to creep along, and surrēptīcius meant furtive or clandestine, so the word describes a way of moving rather than a motive.
Latin
surrēptīcius — furtive, clandestine
English, today
surreptitious
Built from
Borrowed from Latin surrēptīcius (“furtive, clandestine”), from surrēpō (“to creep along”).
Etymology from Wiktionary, CC BY-SA