silo n.

1835, "a pit in the ground or a cavity in rock for storage of green crops," often in reference to other countries, and the word in English is explained at first, 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."

Silo ; a Spanish word, signifying an excavation about fourteen feet deep, for preserving grain. It is best made in marly ground, not too dry. Over the bottom a vaulted dome is built, rising eight and a half feet, and surrounding the tube through which the corn is poured in. The walls of the excavation are lived with straw. [Encyclopædia Americana, Philadelphia, 1832]

By 1867 in reference to airtight above-ground cylindrical structures for storing crops. The meaning "underground housing and launch tube for a guided missile" is attested from 1958.