The Collaborative International Dictionary of English v.0.48:

Pea-jacket \Pea"-jack`et\, Pea jacket \Pea" jack`et\ Pea jacket
\Pea" jack`et\, n. [Prob. fr. D. pij, pije, a coat of a coarse
   woolen stuff.]
   A thick loose double-breasted woolen jacket, or coat,
   commonly worn by sailors in cold weather.

   Syn: pea coat, peacoat, pea jacket.
        [1913 Webster+ WordNet 1.5]

WordNet (r) 3.0 (2006):

pea jacket
    n 1: a sailor's heavy woolen double-breasted jacket [syn: pea
         jacket, peacoat]