The Collaborative International Dictionary of English v.0.48:

Smithsonian \Smith*so"ni*an\ (-s[=o]"n[i^]*an), a.
   Of or pertaining to the Englishman J. L. M. Smithson, or to
   the national institution of learning which he endowed at
   Washington, D. C.; as, the Smithsonian Institution;
   Smithsonian Reports. -- n. The Smithsonian Institution.
   [1913 Webster]