Closing something tightly to prevent anything from entering or leaving, or the act of securing something with a seal.
From Old French 'seel' (to close with a seal), from Latin 'sigillum' (seal or mark), originally used for official documents.
Sealing was how ancient governments authenticated documents—they'd melt wax, press it with a unique seal, and that wax impression proved the document was official and unchanged, making it the original 'security feature!'
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