Luncheon
late 16th century (in the sense ‘thick piece, hunk’): possibly an extension of obsolete lunch ‘thick piece, hunk’, from Spanish lonja ‘slice’.
wiktionary
From earlier lunching, of uncertain origin. Possibly related to lunch, q.v. It is unclear which came first. Possibly influenced by nuncheon(“light snack taken in the afternoon”).
etymonline
luncheon (n.)
"light repast between mealtimes," 1650s (lunching; spelling luncheon by 1706); earlier "thick piece, hunk (of bread)," 1570s (luncheon), which is of uncertain origin. Perhaps it is based on northern English dialectal lunch "hunk of bread or cheese" (1580s; said to be 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 first attested more than a century later. It suggests perhaps an analogy with truncheon, etc., or to simulate a French origin. Especially in reference to an early afternoon meal eaten by those who have a noontime dinner.
