In the Oxford English Dictionary (OED), the concept of sealing one's lips to maintain secrecy is cataloged under the entry for the verb “seal,” specifically under the sense denoting to fasten, close tightly, or render inaccessible. [1]

The structural and historical breakdown of your questions reveals the following:

1. Where is it located?

• Main Entry: It is found under **seal, v.¹ (not under lip or under seal).
• The "Under Seal" Distinction: The OED treats the phrase “under seal” entirely separately as a legal and administrative idiom (e.g., a document or contract executed under an official wax seal). It does not cross-reference the physical binding of lips. [1, 2]

2. What is the first citation?

While the exact phrase "my lips are sealed" gained its widespread modern idiomatic status later, the literal and figurative act of "sealing the lips" for silence has a deep history in the OED:

• The First Textual Citation: The earliest related citation for sealing lips in English literature appears in William Shakespeare’s Henry VI, Part 2 (written c. 1591):

  “Seal up your lips, and give no words but mum.”

• The Modern Idiom: The specific phrasing of modern use—where the lips themselves act as the passive agent ("my lips are sealed")—is often traced forward to the late 18th century, such as in Fanny Burney's 1782 novel Cecilia: “I make it quite a principle to seal my lips...” [1, 2, 3]

3. Etymological Sources: French, Latin, or Italian?

The OED traces the verb seal from Middle English (selen), derived from Anglo-Norman / Old French (seeler), which in turn originates from the classical Latin sigillare (to mark with a device/seal). [1]

However, the idiom of using a "seal" metaphorically for the mouth is an ancient, cross-cultural trope rather than a direct translation loan from just one language:

• Latin Roots: The concept of closing the mouth like a sealed vessel or door is prominent in classical and ecclesiastical Latin (e.g., sigillum silentii—the seal of silence).
• The Romance Language Parallels: Interestingly, while English uses "sealed lips," French and Italian evolved their own distinct idiomatic expressions for the exact same concept:
  • French: Use bouche cousue (literally, "sewed mouth").
  • Italian: Frequently use acqua in bocca (literally, "water in the mouth," implying you can't speak without spilling it). [1]

If you are tracking down a specific historical text or translation, let me know the approximate era or context of the quote you are trying to find, and I can help dig deeper into its linguistic timeline!