"fish-eating marine mammal with flippers; any pinniped not a walrus," Middle English sele, from Old English seolh "seal," from Proto-Germanic *selkhaz (compare Old Norse selr, Swedish sjöl, Danish sæl, Middle Low German sel, Middle Dutch seel, Old High German selah), a word of unknown origin, perhaps a borrowing from Finnic.
Seal point "dark brown marking on a Siamese cat" is recorded from 1934, from the resemblance to the color of seal fur; compare seal brown "rich, dark brown color," which is attested by 1875. Old English seolhbæð, literally "seal's bath," was an Anglo-Saxon kenning for "the sea."
"design stamped on wax," especially an impressed figure attached to a document as evidence of authenticity, c. 1200, sel, sele, from Old French seel, seal "seal on a letter" (Modern French sceau), from Vulgar Latin *sigellum (source of Italian suggello, Spanish sello; also Old Frisian and Middle High German sigel, German Siegel), from Latin sigillum "small picture, engraved figure, seal," diminutive of signum "identifying mark, sign" (see sign (n.)).
An earlier borrowing directly from Latin is represented by Old English insigel. The use for "engraved device of some hard material used for imprinting a seal" (technically a matrix) is by c. 1300. Extended senses are via the notion of a seal used to mark and close a document to insure its secrecy (c. 1300). The meaning "an identifying mark" is from mid-14c.; especially one confirming goods and measures as conforming to standard. The technical sense of "what prevents the escape of a gas or liquid" is by 1853.
c. 1200, selen, "to fasten (a letter, etc.) with or as with a seal, close up with a seal, press a seal on wax," also "place a seal on (a document)," also figurative, "to join together," from seal (n.1) or else from Old French seeler, sealer.
Hence "to conclude, ratify, render official or binding" by affixing seals to it (late 15c.). In reference to jars or other containers, "to close up with wax, lead, cement, etc.," attested from 1660s, from the notion of wax seals on envelopes. In reference to the actions of wood-coatings, "render impervious," by 1940. Related: Sealed; sealing.
Sealing-wax, "soft substance prepared for receiving the impression of a seal," is attested from c. 1300. To seal (one's) lips "be silent" is by 1782. To seal (one's) fate (1799) "decide irrevocably" perhaps reflects the notion of a seal on a warrant of execution.