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1835, from Spanish silo, traditionally derived from Latin sirum (nominative sirus), from Greek siros "a pit to keep corn in." "The change from r to l in Spanish is abnormal and Greek siros was a rare foreign term peculiar to regions of Asia Minor and not likely to emerge in Castilian Spain" [Barnhart]. Alternatively, the Spanish word is from a pre-Roman Iberian language word represented by Basque zilo, zulo "dugout, cave or shelter for keeping grain." Meaning "underground housing and launch tube for a guided missile" is attested from 1958.
in agriculture, airtight structure that encloses and protects silage (q.v.; partially fermented fodder, called haylage if made from grass), keeping it in the succulent and slightly sour condition edible for farm animals.