"design stamped on wax," especially one attached to a document as evidence of authenticity, c. 1200, from Old French seel "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. Technical use, "what prevents the escape of a gas or liquid" is from 1853.
fish-eating mammal with flippers, 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), of unknown origin, perhaps a borrowing from Finnic. Seal point "dark brown marking on a Siamese cat" is recorded from 1934, from the dark brown color of seal fur; compare seal brown "rich, dark brown color," by 1875. Old English seolhbæð, literally "seal's bath," was an Anglo-Saxon kenning for "the sea."
"to fasten with (or as with) a seal," c. 1200, from seal (n.1). Meaning "to place a seal on (a document)" is recorded from mid-14c.; hence "to conclude, ratify, render official" (late 15c.). Sense of "to close up with wax, lead, cement, etc." is attested from 1660s, from the notion of wax seals on envelopes. In reference to the actions of wood-coatings, 1940. Related: Sealed; sealing. Sealing-wax is attested from c. 1300. To seal (one's) fate (1799) probably reflects the notion of a seal on an execution warrant.