half seas over

<. Meaning "large quantity" (of anything) is from c.1200. Meaning "dark area of the moon's surface" is attested from 1660s (see mare (n.2)).

Germanic languages also use the general Indo-European word (represented by English mere (n.)), but have no firm distinction between “sea” and “lake,” either by size, by inland or open, or by salt vs. 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 (e.g. Gothic saiws “lake,” marei “sea;” but Dutch zee “sea,” meer “lake”). Cf. also Old Norse sær “sea,” but Danish sø, usually “lake” but “sea” in phrases. German See is “sea” (fem.) or “lake” (masc.). The single Old English word sæ glosses Latin mare, aequor, pontus, pelagus, and marmor.

Phrase sea change “transformation” is attested from 1610, first in Shakespeare (“The Tempest,” I.ii). Sea anemone is from 1742; sea legs is from 1712; sea level from 1806; sea urchin from 1590s. At sea in the figurative sense of “perplexed” is attested from 1768, from literal sense of “out of sight of land” (c.1300).

1. The continuous body of salt water that covers most of the Earth’s surface.
2. A region of water within an ocean and partly enclosed by land, such as the North Sea. See Note at ocean.
3. A large body of either fresh or salt water that is completely enclosed by land, such as the Caspian Sea.
4. Astronomy A mare.

In addition to the idiom beginning with sea

• sea legs
• seal of approval
• seal off
• seal one’s fate

also see:

• at sea
• between a rock and a hard place (devil and the deep blue sea)
• high seas
• not the only fish in the sea
• put out (to sea)