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Surreptitious describes something done secretly and quickly, so that other people will not notice. The key idea is hiding the action while you are doing it, usually because you are not supposed to be doing it — a surreptitious look at your phone under the desk, a surreptitious photo of someone, a surreptitious glance at your classmate's answers. It is nearly always used for small actions: looks, glances, movements, quick photos, passing a note. You would not normally call a big crime surreptitious; for that you would say secret or covert. The feeling is guilty rather than seriously criminal — often it is used with a smile. It is a formal, written word; in speech people are more likely to say "sneaky" or "on the quiet" or just "He looked at his phone secretly." Notice it usually sits in front of a noun (a surreptitious glance) rather than after "be" — "His glance was surreptitious" is possible but sounds stiff.
偷偷摸摸的、鬼鬼祟祟的（做小動作不想被發現）
組成From Latin surrepticius "done by stealth", from surripere / surrepere — sub- "under, from below" (changed to sur- before r) + rapere "to snatch" or repere "to creep". So the picture is of creeping up from underneath, or snatching something out from under someone.
原始詞典資料1 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
surreptitiousadj/ˌsʌɹɪpˈtɪʃəs/
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Stealthy, furtive, well hidden, covert (especially movements).
- He read the letter aloud. Sophia listened with the studied air of one for whom, even in these days, a title possessed some surreptitious allurement.
- It is also worth noting the case law on prisoners' correspondence which establishes that interception of a person's communications need not be surreptitious in order to amount to an interference with respect to Art 8 (1) [ECHR].
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。