The Collaborative International Dictionary of English v.0.48:

Decker \Deck"er\ (d[e^]k"[~e]r), n.
   1. One who, or that which, decks or adorns; a coverer; as, a
      table decker.
      [1913 Webster]

   2. A vessel or vehicle which has a deck or decks; -- used
      esp. in composition; as, a single-decker; a three-decker;
      a double-decker bus.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

Decker
    n 1: English dramatist and pamphleteer (1572-1632) [syn:
         Dekker, Decker, Thomas Dekker, Thomas Decker]
    2: (often used in combinations) something constructed with
       multiple levels; "they rode in a double-decker bus"