The Collaborative International Dictionary of English v.0.48:

Blackguard \Black"guard`\, v. t. [imp. & p. p. Blackguarded;
   p. pr. & vb. n. Blackguarding.]
   To revile or abuse in scurrilous language. --Southey.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Blackguard \Black"guard\, a.
   Scurrilous; abusive; low; worthless; vicious; as, blackguard
   language.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Blackguard \Black"guard\ (bl[a^]g"g[aum]rd), n. [Black + guard.]
   1. The scullions and lower menials of a court, or of a
      nobleman's household, who, in a removal from one residence
      to another, had charge of the kitchen utensils, and being
      smutted by them, were jocularly called the "black guard";
      also, the servants and hangers-on of an army. [Obs.]
      [1913 Webster]

            A lousy slave, that . . . rode with the black guard
            in the duke's carriage, 'mongst spits and dripping
            pans.                                 --Webster
                                                  (1612).
      [1913 Webster]

   2. The criminals and vagrants or vagabonds of a town or
      community, collectively. [Obs.]
      [1913 Webster]

   3. A person of stained or low character, esp. one who uses
      scurrilous language, or treats others with foul abuse; a
      scoundrel; a rough.
      [1913 Webster]

            A man whose manners and sentiments are decidedly
            below those of his class deserves to be called a
            blackguard.                           --Macaulay.
      [1913 Webster]

   4. A vagrant; a bootblack; a gamin. [Obs.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

blackguard
    n 1: someone who is morally reprehensible; "you dirty dog" [syn:
         cad, bounder, blackguard, dog, hound, heel]
    v 1: subject to laughter or ridicule; "The satirists ridiculed
         the plans for a new opera house"; "The students poked fun
         at the inexperienced teacher"; "His former students roasted
         the professor at his 60th birthday" [syn: ridicule,
         roast, guy, blackguard, laugh at, jest at, rib,
         make fun, poke fun]
    2: use foul or abusive language towards; "The actress abused the
       policeman who gave her a parking ticket"; "The angry mother
       shouted at the teacher" [syn: abuse, clapperclaw,
       blackguard, shout]

The Devil's Dictionary (1881-1906):

BLACKGUARD, n.  A man whose qualities, prepared for display like a box
of berries in a market -- the fine ones on top -- have been opened on
the wrong side.  An inverted gentleman.