Source : Webster's Revised Unabridged Dictionary (1913)

Mortar \Mor"tar\, n. [OE. mortier, F. mortier, L. mortarium
   mortar, a large basin or trough in which mortar is made, a
   mortar (in sense 1, above). See 1st {Mortar}.] (Arch.)
   A building material made by mixing lime, cement, or plaster
   of Paris, with sand, water, and sometimes other materials; --
   used in masonry for joining stones, bricks, etc., also for
   plastering, and in other ways.

   {Mortar bed}, a shallow box or receptacle in which mortar is
      mixed.

   {Mortar board}.
   (a) A small square board with a handle beneath, for holding
       mortar; a hawk.
   (b) A cap with a broad, projecting, square top; -- worn by
       students in some colleges. [Slang]