horse's legs; minor actor playing a horse's legs
Literally 'horse's legs'. Idiomatically refers to a low-ranking actor in traditional theater who plays the rear legs of a stage horse, and by extension any minor or untalented performer relegated to such roles.
He's still a novice and plays the horse's legs on stage.
馬の足役から這い上がって、ついに主役を射止めた。
He worked his way up from playing the horse's legs and finally landed the lead role.