The Collaborative International Dictionary of English v.0.48:

Orlop \Or"lop\, n. [D. overloop the upper deck, lit., a running
   over or overflowing, fr. overloopen to run over. See Over,
   and Leap, and cf. Overloop.] (Naut.)
   The lowest deck of a vessel, esp. of a ship of war,
   consisting of a platform laid over the beams in the hold, on
   which the cables are coiled.
   [1913 Webster] Ormuzd
   Ormazd

WordNet (r) 3.0 (2006):

orlop
    n 1: the fourth or lowest deck [syn: orlop deck, orlop,
         fourth deck]