sea-horse (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 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.

The 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).

The sea, the most intact and ancient thing on the globe.

    Everything it touches is a ruin; everything it abandons is new.

[Paul Valéry, "Notebook" entry, 1921, transl. Nathaniel Brudavsky-Brody]

"solidungulate perissodactyl mammal of the family Equidæ and genus Equus" [Century Dictionary], Old English hors "horse," from Proto-Germanic *harss- (source also of Old Norse hross, Old Frisian, Old Saxon hors, Middle Dutch ors, Dutch ros, Old High German hros, German Roß "horse"), of unknown origin. By some, connected to PIE root *kers- "to run," source of Latin currere "to run." Boutkan prefers the theory that it is a loan-word from an Iranian language (Sarmatian) also borrowed into Uralic (compare Finnish varsa "foal"),

The usual Indo-European word is represented by Old English eoh, Greek hippos, Latin equus, from PIE root *ekwo-. Another Germanic "horse" word is Old English vicg, from Proto-Germanic *wegja- (source also of Old Frisian wegk-, Old Saxon wigg, Old Norse vigg), which is of uncertain origin. In many other languages, as in English, this root has been lost in favor of synonyms, probably via superstitious taboo on uttering the name of an animal so important in Indo-European religion. For the Romanic words (French cheval, Spanish caballo) see cavalier (n.); for Dutch paard, German Pferd, see palfrey; for Swedish häst, Danish hest see henchman. As plural Old English had collective singular horse as well as horses, in Middle English also sometimes horsen, but horses has been the usual plural since 17c.

Used at least since late 14c. of various devices or appliances which suggest a horse (as in sawhorse), typically in reference to being "that upon which something is mounted." For sense of "large, coarse," see horseradish. Slang use for "heroin" is attested by 1950. To ride a horse that was foaled of an acorn (1670s) was through early 19c. a way to say "be hanged from the gallows." Horse latitudes first attested 1777, the name of unknown origin, despite much speculation. Horse-pistol, "large one-handed pistol used by horseback riders," is by 1704. A dead horse as a figure for something that has ceased to be useful is from 1630s; to flog a dead horse "attempt to revive interest in a worn-out topic" is from 1864.

HORSEGODMOTHER, a large masculine wench; one whom it is difficult to rank among the purest and gentlest portion of the community. [John Trotter Brockett, "A Glossary of North Country Words," 1829]

The term itself is attested from 1560s. The horse's mouth as a source of reliable information is from 1921, perhaps originally of racetrack tips, from the fact that a horse's age can be determined accurately by looking at its teeth. To swap horses while crossing the river (a bad idea) is from the American Civil War and appears to have been originally one of Abe Lincoln's stories. Horse-and-buggy meaning "old-fashioned" is recorded from 1926 slang, originally in reference to a "young lady out of date, with long hair." To hold (one's) horses "restrain one's enthusiasm, be patient" is from 1842, American English; the notion is of keeping a tight grip on the reins.

"large pinniped carnivorous mammal" (the males are noted for their  enormous tusk-like canine teeth), 1650s, from Dutch walrus, which was probably a folk-etymology alteration (by influence of Dutch walvis "whale" and ros "horse") of a Scandinavian word, such as Old Norse rosmhvalr "walrus," hrosshvalr "a kind of whale," or rostungr "walrus." Old English had horschwæl, and later morse, from Lapp morsa or Finnish mursu, which ultimately might be the source, much garbled, of the first element in Old Norse rosmhvalr.