A tailcoat (also known as a cutaway) is a gentleman's long coat, cut away at the front and split at the back, giving the impression of a tail. The tailcoat evolved in England among the horse-riding gentry who found the conventional long coat too awkward for horse riding, and so had the lower front part removed and the back split to allow sitting astride a horse.
The tailcoat became unfashionable, giving way to the lounge-suit, which in turn was pushed out of fashion by the hacking-jacket worn by high-status horse riders, which today is known as a 'sports jacket'. Research Tailcoat
 
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