To fail badly. “Cropper” comes from a horse's croup or crupper, the part of the animal's back behind the saddle. Someone who parted company from his horse (an involuntary dismount, so to speak) was said to fall “neck and crop.” That became “come a-cropper,” first appearing in the foxhunting author Robert S. Surtees' 1858 novel Ask Mamma: [He] “rode at an impracticable fence, and got a cropper for his pains.” The phrase was picked up and applied to any misadventure, equestrian or otherwise.