half seas over Look up half seas over at Dictionary.com
slang for "drunk," 1736, sometimes said to be from notion of a ship heavy-laden and so low in the water that small waves (half seas) wash over the deck. This suits the sense, but the phrase is not recorded in this alleged literal sense. Half seas over "halfway across the sea" is recorded from 1550s, however, and it was given a figurative extension to "halfway through a matter" by 1690s. What drunkenness is halfway to is not clear.