The Collaborative International Dictionary of English v.0.48:

Argonaut \Ar"go*naut\, n. [L. Argonauta, Gr. ?; ? + ? sailor, ?
   ship. See Argo.]
   1. Any one of the legendary Greek heroes who sailed with
      Jason, in the Argo, in quest of the Golden Fleece.
      [1913 Webster]

   2. (Zool.) A cephalopod of the genus Argonauta.
      [1913 Webster]

   2. One of those who went to California in search of gold
      shortly after it was discovered there in 1848. [U. S.]
      --Bret Harte.
      [1913 Webster]

            The "Argonauts of '49" were a strong, self-reliant,
            generous body of men.                 --D. S.
                                                  Jordan.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

argonaut
    n 1: someone engaged in a dangerous but potentially rewarding
         adventure
    2: (Greek mythology) one of the heroes who sailed with Jason in
       search of the Golden Fleece
    3: cephalopod mollusk of warm seas whose females have delicate
       papery spiral shells [syn: paper nautilus, nautilus,
       Argonaut, Argonauta argo]