come a cropper

英国人で "its" と "it's" の使い方を間違える人が多いと言うのはちょっと信じがたい話です。
We had been taught Latin, French and German grammar; but English grammar was something we felt we were expected to infer from our reading - which is doubtless why I came a cropper over "its" and "it's". Like many uninstructed people, I surmised that, if there was a version of "its" with an apostrophe before the "s", there was somehow logically bound to be a version of "its" with an apostrophe after the "s" as well.
上の引用文の "came a cropper" の個所が分かりません。
・Oxford English Dictionary: Suffer a defeat or disaster: ‘the club’s challenge for the championship has come a cropper’
・Collins Dictionary: 1. to fall heavily 2.to fail completely: Throughout the centuries, invading armies have spectacularly come a cropper in this strategically important country.
この慣用句の由来がWord Detectiveに書かれていました。全文は長いので一部のみ引用します。
Thank heavens she appears to be mildly psychic. "To come a cropper" does indeed come from the world of horse riding and racing. The original phrase was "neck and crop," describing a fall from a horse where the rider is thrown headlong over the horse's head. The most common occasion for this sort of extremely unpleasant accident is when the horse stops short of a jump, as in a steeplechase, but the rider keeps going. "Neck and crop" itself refers to the horse's head, "crop" being another word for "throat."
As a metaphor for failure, "come a cropper" graduated from the world of equestrian mishaps to general use in the mid-19th century. Anthony Trollope illustrated the new sense perfectly in his 1874 novel "The Way We Live Now": "He would be 'coming a cropper rather' were he to marry Melmotte's daughter for her money, and then find that she had got none."
全文を読みたい方はWord Detectiveの次のURLに行ってください。"http://www.word-detective.com/030698.html#cropper"