Source : Webster's Revised Unabridged Dictionary (1913)

Blackmail \Black"mail`\, v. t. [imp. & p. p. {Blackmailed}; p.
   pr. & vb. n. {Blackmailing}.]
   To extort money from by exciting fears of injury other than
   bodily harm, as injury to reputation, distress of mind, etc.;
   as, to blackmail a merchant by threatening to expose an
   alleged fraud. [U. S.]