The Collaborative International Dictionary of English v.0.48:

Paillasse \Pail*lasse"\ (?; F. ?), n. [F., fr. paille straw. See
   Pallet a bed.]
   An under bed or mattress of straw. [Written also
   palliasse.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

paillasse
    n 1: mattress consisting of a thin pad filled with straw or
         sawdust [syn: paillasse, palliasse]