The Collaborative International Dictionary of English v.0.48:

Boottree \Boot"tree`\, n. [Boot + tree wood, timber.]
   An instrument to stretch and widen the leg of a boot,
   consisting of two pieces, together shaped like a leg, between
   which, when put into the boot, a wedge is driven.
   [1913 Webster]

         The pretty boots trimly stretched on boottrees.
                                                  --Thackeray.
   [1913 Webster]