noun
Informal term for a close friend, often used among men. Slightly old-fashioned; 親友 or 仲間 are more common today.
彼とは子供の頃からの合口だ。
He's been a close friend of mine since childhood.
noun
Refers to a short-bladed weapon, often a stiletto or dagger. The kanji 合口 literally means 'matching mouth', referring to the hilt fitting snugly into the scabbard. See also 匕首.
See also: 匕首
侍は懐に合口を忍ばせていた。
The samurai concealed a dagger in his breast pocket.
noun
unbalanced head-to-head record (sumo)
Sumo term for a win-loss record between two wrestlers that is heavily lopsided, e.g., one wrestler has many more wins than the other in their match history.
この二人は合口で、横綱が十勝一敗と圧倒している。
These two have an unbalanced record, with the yokozuna dominating at ten wins to one loss.
The reading あいくち is a compound of 合い (ai, 'mutual') and 口 (kuchi, 'mouth'), literally 'matching mouths'. The 'dagger' sense comes from the hilt fitting snugly into the scabbard without a guard. The 'chum' sense is a metaphorical extension of 'fitting well together'. The sumo sense is a specialized use of the same metaphor.