The Collaborative International Dictionary of English v.0.48:

paperknife \pa"per*knife`\ (p[=a]"p[~e]r*n[imac]f`), n.
   A dull knife used to cut open the envelopes in which letters
   are mailed or to slit uncut pages of books.

   Syn: letter opener.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

paperknife
    n 1: dull knife used to cut open the envelopes in which letters
         are mailed or to slit uncut pages of books [syn: letter
         opener, paper knife, paperknife]