c. 1600, "kind of lobster," from sea + lion. Later the name of a fabulous animal (in heraldry, etc.), 1660s. Applied from 1690s to various species of large eared seals. As code name for the planned German invasion of Britain, it translates German Seelöwe, announced by Hitler July 1940, scrubbed October 1940.
Entries linking to sea-lion
Middle English se, seo, from Old English sæ,"sheet of water, sea, lake, pool," from Proto-Germanic *saiwa- (source also of Old Saxon seo, Old Frisian se, Middle Dutch see, Dutch zee, German See, Swedish sjö), of unknown origin, outside connections "wholly doubtful" [Buck], and an IE etymon "has generally been doubted" [Boutkan]. The meaning "any great mass or large quantity" (of anything) is from c. 1200.
Germanic languages also use the more general Indo-European word (represented by English mere (n.1)), but have no firm distinction between "sea" and "lake," either large or small, by inland or open, salt or fresh. This may reflect the Baltic geography where the languages are thought to have originated. The two words are used more or less interchangeably in Germanic, and exist in opposite senses (such as Gothic saiws "lake, marshland," marei "sea;" but Dutch zee "sea," meer "lake"). Compare also Old Norse sær "sea," but Danish sø, usually "lake" but "sea" in phrases. German See is "sea" (fem.) or "lake" (masc.).
Boutkan writes that the sea words in Germanic likely were originally "lake," and the older word for "sea" is represented by haff. The single Old English word sæ glosses Latin mare, aequor, pontus, pelagus, and marmor. The range in the Old English word included "the expanse of salt water that covers much of the world" to individual great, distinctly limited bodies of water; it also was used of inland seas, bogs, lakes, rivers, and the Bristol Channel.
Meaning "dark area of the moon's surface" is attested from 1660s (see mare (n.2)); before the invention of telescopes they were supposed to be water. The phrase sea change "transformation," literally "a change wrought by the sea," is attested from 1610, first in Shakespeare ("The Tempest," I.ii). Sea legs, humorous colloquial term implying ability to walk on a ship's deck when she is pitching or rolling is from 1712. At sea in the figurative sense of "perplexed" is attested from 1768, from literal sense (in reference to ships) of "out of sight of land" (c. 1300).
late 12c., from Old French lion "lion," also figuratively "hero" (12c.), from Latin leonem (nominative leo) "lion; the constellation Leo," from Greek leon (genitive leontos), a word from a non-Indo-European language, perhaps Semitic (compare Hebrew labhi "lion," plural lebaim; Egyptian labai, lawai "lioness"). Old English had the word straight from Latin as leo (Anglian lea).
The Latin word was borrowed throughout Germanic (compare Old Frisian lawa; Middle Dutch leuwe, Dutch leeuw; Old High German lewo, German Löwe); it is also found in most other European languages, often via Germanic (Old Church Slavonic livu, Polish lew, Czech lev, Old Irish leon, Welsh llew).
Extended 17c. to American big cats. Sometimes used ironically of other animals (for example Cotswold lion "sheep" (16c.; lyons of Cotteswold is from mid-15c.). In early 19c., to avoid advertising breaches of the game laws, hare, when served as food was listed as lion.
Paired alliteratively with lamb since late 14c. Used figuratively from c. 1200 in English of lion-like persons, in an approving sense, "one who is fiercely brave," and a disapproving one, "tyrannical leader, greedy devourer." Lion-hearted is from 1708. Lion's share "the greatest portion" is attested from 1701. The image of the lion's mouth as a place of great danger is from c. 1200.
Lowse me, lauerd, ut of þe liunes muð. ["St. Margaret of Antioch," c. 1200]