The Collaborative International Dictionary of English v.0.48:

Pooh-pooh \Pooh`-pooh"\, v. t.
   To make light of; to treat with derision or contempt, as if
   by saying pooh! pooh! [Colloq.] --Thackeray.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pooh-pooh
    v 1: express contempt about
    2: reject with contempt; "She spurned his advances" [syn:
       reject, spurn, freeze off, scorn, pooh-pooh,
       disdain, turn down]