Word Origin & History
luncheon1580, nonechenche "light mid-day meal," from none "noon" + schench "drink," from O.E. scenc, from scencan "pour out." Altered by northern Eng. dial. lunch "hunk of bread or cheese" (1590), which probably is from Sp. lonja "a slice," lit. "loin." When it first appeared, luncheon meant "thick piece, hunk;"
sense of "light repast between mealtimes" is from 1650s, especially in reference to an early afternoon meal eaten by those who have a noontime
dinner. Slang phrase out to lunch "insane, stupid, clueless" first recorded 1955, on notion of being "not there."