• Webster's Revised Unabridged Dictionary
    • n Sidebone (Far) A morbid growth or deposit of bony matter and at the sides of the coronet and coffin bone of a horse.
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Century Dictionary and Cyclopedia
    • n sidebone The hip-bone.
    • n sidebone An abnormal ossification of the lateral elastic cartilage in a horse's foot. Side-bones occur chiefly in the fore feet of draft-horses, and are an occasional cause of lameness.
    • n sidebone The disease or disordered condition in horses which causes the lateral cartilages above the heels to ossify. See the quotation under ring-bone.
    • n sidebone In carving, either half, right or left, of the pelvis of a fowl, without the sacrarium; the hip-bone or haunch-bone, consisting of the coalesced ilium, ischium, and pubis, easily separated from the backbone. The so-called “second joint” of carvers is articulated at the hip-joint with the side-bone. The meat on the outside of the side-bone includes the piece called the oyster, and the concavity of the bone holds a dark mass of flesh (the kidney). See cuts under sacrarium.
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In literature:

The tendons may soften and rupture, the hoof may slough off, quittors develop, or sidebones and ringbones grow.
"Special Report on Diseases of the Horse" by United States Department of Agriculture
Turn the rump from you, and very neatly take off the two sidebones, and the whole will be done.
"The Cook and Housekeeper's Complete and Universal Dictionary; Including a System of Modern Cookery, in all Its Various Branches," by Mary Eaton