The Collaborative International Dictionary of English v.0.48:

Owler \Owl"er\, n. [From Owl, v. i.]
   One who owls; esp., one who conveys contraband goods. See
   Owling, n. [Obs. or Prov. Eng.] --T. Brown.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

OWLER, Eng. law. One guilty of the offence of owling.