also: くそまじめ · クソマジメ
na-adjective
overly serious; serious to a fault; humorless
Describes a person or attitude that is excessively earnest, rigid, or lacking in humor, often with a negative or mocking tone. The prefix クソ adds a vulgar, emphatic nuance similar to 'damn' or 'stupidly'.
彼はクソ真面目すぎて、冗談が通じない。
He's so overly serious that jokes don't get through to him.
クソ真面目な顔で変なことを言うから、笑って
He said something weird with a dead-serious face, so I burst out laughing.
All-katakana spelling; used for strong emphasis or in informal writing.
Compound of the vulgar intensifier クソ (kuso, 'damn') and 真面目 (majime, 'serious'). The prefix クソ is used here to express an extreme or excessive degree, often with a derogatory nuance.