A possible Spanish origin for "lunch"
The possible Spanish origin of lunch is lonja 'slice' which comes from French longe 'long' and ultimately from Latin longus 'long'. The original and obsolete sense of lunch from OED "A piece, a thick piece; a hunch or hunk." is directly related to this origin.
The etymology of "lunch" from OED:
Perhaps evolved < lump n.1, on the analogy of the apparent relation between hump and hunch, bump and bunch. Compare ‘Lounge, a large lump, as of bread or cheese’ (Brockett N. Country Words, ed. 2, 1829).
It is curious that the word first appears as a rendering of the (at that time) like-sounding Spanish lonja slice of ham. luncheon n., commonly believed to be a derivative of lunch , occurs in our quots. 11 years earlier, with its present spelling. In sense 2 lunch was an abbreviation of luncheon, first appearing about 1829, when it was regarded either as a vulgarism or as a fashionable affectation.
The second and usual sense of "lunch" in OED:
a. A synonym of luncheon n. 2. (Now the usual word except in specially formal use, though formerly objected to as vulgar.) Also: a light meal at any time of the day.
b. out to lunch: insane; stupid, unaware; socially unacceptable. North American slang.
Eric Partridge strongly believes that the origin of lunch and luncheon is the Spanish lonja and says that lunch is earlier than luncheon. Here is a relevant excerpt from his book Word, Words Words!:
Ernest Weekley and Samuel Johnson mention the Spanish lonja for the origin of lunch in their etymology dictionaries as well.
Bonus:
The similar word lounge appears to share the same origin as lunch. OED mentions the North England dialect word lounge (meaning a large lump, as of bread or cheese) which is connected to the obsolete sense of lunch and to Spanish lonja.
OED says the noun lounge is from the verb lounge, and the origin is mentioned as "Of obscure origin; perhaps suggested by lungis n.". Lungis shares the same origin as Spanish lonja.
The etymology of lungis from OED:
< Old French longis < Latin Longīnus apocryphal name of the centurion who pierced Jesus with a spear, by popular etymology associated with Latin longus long.