The Collaborative International Dictionary of English v.0.48:

Why-not \Why"-not`\, n.
   A violent and peremptory procedure without any assigned
   reason; a sudden conclusive happening. [Obs.]
   [1913 Webster]

         When the church
         Was taken with a why-not in the lurch.   --Hudibras.
   [1913 Webster]

         This game . . . was like to have been lost with a
         why-not.                                 --Nugae Antiq.
   [1913 Webster]