Etymology
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ladder (n.)

Old English hlæder "ladder, steps," from Proto-Germanic *hlaidri (source also of Old Frisian hledere, Middle Dutch ledere, Old High German leitara, German Leiter), from suffixed form of PIE root *klei- "to lean" (source also of Greek klimax "ladder"). In late Old English, rungs were læddrestæfæ and the side pieces were ledder steles. The belief that bad things happen to people who walk under ladders is attested from 1787, but its origin likely is more scientific than superstitious.

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sea (n.)

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 , 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 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).

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step-ladder (n.)

also stepladder, one with flat steps instead of rungs, 1728, from step (n.) + ladder.

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ladder-back (adj.)

1898 as a type of chair, from ladder (n.) + back (n.).

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sea-salt (n.)

"common salt obtained by evaporation of sea water," c. 1600, from sea + salt (n.).

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sea-going (adj.)

"designed or fit for going to sea," 1829; see sea + going.

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sea-dog (n.)

1590s, "harbor seal," from sea + dog (n.). Also "pirate" (1650s). Meaning "old seaman, sailor who has been long afloat" is attested by 1823. In Middle English sea-hound was used of the walrus and the beaver.

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sea-floor (n.)

1832, from sea + floor (n.). Old English had -grund; Middle English had sea-bottom (c. 1400).

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sea-anemone (n.)

by 1742; see sea + anemone. Another name for it was sea-pudding (1750).

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sea-horse (n.)

late 15c., "walrus" (apparently), from sea + horse (n.); compare walrus. Also in heraldry as a fabulous animal with the foreparts of a horse and the tail of a fish. Main modern sense in zoology is attested from 1580s.

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