The Collaborative International Dictionary of English v.0.48:

Slape \Slape\, a. [Icel. sleipr slippery; akin to E. slip.]
   Slippery; smooth; crafty; hypocritical. [Prov. Eng.]
   [1913 Webster]

   Slape ale, plain ale, as opposed to medicated or mixed
      ale. [Prov. Eng.]
      [1913 Webster]