The Collaborative International Dictionary of English v.0.48:

Backbond \Back"bond`\ (b[a^]k"b[o^]nd`), n. [Back, adv. + bond.]
   (Scots Law)
   An instrument which, in conjunction with another making an
   absolute disposition, constitutes a trust.
   [1913 Webster]

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

BACK-BOND. A bond given by one to a surety, to indemnify such surety in case
of loss. In Scotland, a back-bond is an instrument which, in conjunction
with another which gives an absolute disposition, constitutes a trust. A
declaration of trust.