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"light repast between mealtimes," 1650s (lunching; spelling luncheon by 1706); earlier "thick piece, hunk," 1570s (luncheon), of uncertain origin. Perhaps northern English dialectal lunch "hunk of bread or cheese" (1580s; probably from Spanish lonja "a slice," literally "loin"), blended with or influenced by nuncheon (Middle English nonechenche, mid-14c.) "light mid-day meal," from none "noon" (see noon) + schench "drink," from Old English scenc, from scencan "pour out."
Despite the form lunching in the 1650s source OED discounts that it possibly could be from lunch (v.), which is much later. It suggests perhaps an analogy with truncheon, etc. Especially in reference to an early afternoon meal eaten by those who have a noontime dinner.