also: いっぽんぜおい
noun
one-armed shoulder throw
A throwing technique in judo and sumo where the attacker grips one arm and throws the opponent over the shoulder. The term is also used metaphorically for a decisive move or winning argument.
彼は一本背負いで相手を投げた。
He threw his opponent with a one-armed shoulder throw.
議論で彼の一本背負いにやられた。
I was defeated by his decisive argument (lit. one-armed shoulder throw).
背負い投げ is a general shoulder throw, while 一本背負い specifically involves gripping one arm.
Compound of 一本 (ippon, 'one arm' or 'one point') and 背負い (seoi, 'carrying on the back'), referring to the technique of throwing an opponent over the shoulder while holding one arm.