deadly serious; deadly earnest
Emphasizes extreme seriousness, often with a nuance of being overly or comically earnest. Used for people, expressions, or attitudes.
He told a joke with a dead serious face.
You don't have to think about it so deadly seriously.
真面目 means 'serious' or 'earnest' in a neutral sense, while 大真面目 intensifies it to 'deadly serious' or 'overly earnest'.
Standard kanji spelling.
Mixed kanji-kana spelling, common in casual writing.
Katakana spelling used for emphasis or in informal contexts.
Compound of 大 (おお, 'big, great') + 真面目 (まじめ, 'serious, earnest'). The prefix 大 intensifies the meaning.