boottree

from The Century Dictionary.

• noun An instrument consisting of two wooden blocks, constituting a front and a rear portion, which together form the shape of the leg and foot, and are inserted into a boot and then forced apart by a wedge for the purpose of stretching it.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a device, shaped like a divided leg with a wedge, used to stretch the leg of a boot