The Collaborative International Dictionary of English v.0.48:

Boss \Boss\ (b[o^]s; 115), n.; pl. Bosses (-[e^]z). [OE. boce,
   bose, boche, OF. boce, boche, bosse, F. bosse, of G. origin;
   cf. OHG. b[=o]zo tuft, bunch, OHG. b[=o]zan, MHG. b[^o]zen,
   to beat. See Beat, and cf. Botch a swelling.]
   1. Any protuberant part; a round, swelling part or body; a
      knoblike process; as, a boss of wood.
      [1913 Webster]

   2. A protuberant ornament on any work, either of different
      material from that of the work or of the same, as upon a
      buckler or bridle; a stud; a knob; the central projection
      of a shield. See Umbilicus.
      [1913 Webster]

   3. (Arch.) A projecting ornament placed at the intersection
      of the ribs of ceilings, whether vaulted or flat, and in
      other situations.
      [1913 Webster]

   4. [Cf. D. bus box, Dan. b["o]sse.] A wooden vessel for the
      mortar used in tiling or masonry, hung by a hook from the
      laths, or from the rounds of a ladder. --Gwilt.
      [1913 Webster]

   5. (Mech.)
      (a) The enlarged part of a shaft, on which a wheel is
          keyed, or at the end, where it is coupled to another.
      (b) A swage or die used for shaping metals.
          [1913 Webster]

   6. A head or reservoir of water. [Obs.]
      [1913 Webster]

Easton's 1897 Bible Dictionary:

Bosses
   the projecting parts of a shield (Job 15:26). The Hebrew word
   thus rendered means anything convex or arched, and hence the
   back, as of animals.