The Collaborative International Dictionary of English v.0.48:

Silo \Si"lo\, n. [F.]
   A pit or vat for packing away green fodder for winter use so
   as to exclude air and outside moisture. See Ensilage.
   [1913 Webster]

WordNet (r) 3.0 (2006):

silo
    n 1: a cylindrical tower used for storing silage
    2: military installation consisting of an underground structure
       where ballistic missiles can be stored and fired

The Free On-line Dictionary of Computing (19 January 2023):

silo

   The FIFO input-character buffer in an EIA-232 serial line
   card.  So called from DEC terminology used on DH and DZ line
   cards for the VAX and PDP-11, presumably because it was a
   storage space for fungible stuff that went in at the top and
   came out at the bottom.


The Jargon File (version 4.4.7, 29 Dec 2003):

silo
 n.

    The FIFO input-character buffer in an RS-232 line card. So called from DEC
     terminology used on DH and DZ line cards for the VAX and PDP-11,
    presumably because it was a storage space for fungible stuff that went in
    at the top and came out at the bottom.