cart-horse; workhorse
Refers literally to a horse that pulls a cart or carriage. Often used figuratively for a person who works tirelessly.
In the past, cart-horses carried goods.
wholeheartedly; with undivided attention
Used in the pattern 馬車馬のように to describe working single-mindedly and tirelessly, like a cart-horse.
He works like a horse.
Compound of 馬車 (ばしゃ, 'horse-drawn carriage') and 馬 (うま, 'horse'). The figurative sense derives from the image of a horse tirelessly pulling a carriage.