The Collaborative International Dictionary of English v.0.48:

Lubber \Lub"ber\, n. [Cf. dial. Sw. lubber. See Looby, Lob.]
   A heavy, clumsy, or awkward fellow; a sturdy drone; a clown.
   [1913 Webster]

         Lingering lubbers lose many a penny.     --Tusser.
   [1913 Webster]

   Land lubber, a name given in contempt by sailors to a
      person who lives on land.

   Lubber grasshopper (Zool.), a large, stout, clumsy
      grasshopper; esp., Brachystola magna, from the Rocky
      Mountain plains, and Romalea microptera, which is
      injurious to orange trees in Florida.

   Lubber's hole (Naut.), a hole in the floor of the "top,"
      next the mast, through which sailors may go aloft without
      going over the rim by the futtock shrouds. It is
      considered by seamen as only fit to be used by lubbers.
      --Totten.

   Lubber's line, Lubber's point, or Lubber's mark, a line
      or point in the compass case indicating the head of the
      ship, and consequently the course which the ship is
      steering.
      [1913 Webster]

WordNet (r) 3.0 (2006):

lubber
    n 1: an awkward stupid person [syn: lout, clod,
         stumblebum, goon, oaf, lubber, lummox, lump,
         gawk]
    2: an inexperienced sailor; a sailor on the first voyage [syn:
       landlubber, lubber, landsman]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "lubber":
   blockhead, blunderer, blunderhead, boor, botcher, bumbler, bumpkin,
   bungler, clock watcher, clod, clodhopper, clodknocker, clot, clown,
   dallier, dawdle, dawdler, diddler, dillydallier, do-nothing,
   dolittle, dolt, doodler, faineant, fumbler, gawk, gawky,
   gentleman of leisure, goldbrick, goldbricker, goof-off, gowk,
   hayseed, hick, idler, klutz, laggard, landlubber, lingerer, loafer,
   lobster, loiterer, loller, looby, lotus-eater, lounger, lout, lug,
   lummox, lump, mope, moper, oaf, ox, palooka, polliwog, potterer,
   putterer, rube, slouch, sloucher, slubberer, slug, sluggard,
   stick-in-the-mud, time killer, time waster, trifler,
   waiter on Providence, yokel