The Collaborative International Dictionary of English v.0.48:

Cautioner \Cau"tion*er\, n.
   1. One who cautions or advises.
      [1913 Webster]

   2. (Scots Law) A surety or sponsor.
      [1913 Webster]

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

CAUTIONER, Scotch law, contracts. One who becomes bound as caution or surety
for another, for the performance of any obligation or contract contained in
a deed.