very small breasts; flat chest
Colloquial and often used in casual or humorous contexts. Compare with 微乳 (びにゅう), which is a more neutral term for small breasts.
See also: 微乳
She is self-conscious about her very small breasts.
When I searched for 'naichichi' online, all sorts of information came up.
Kana-mixed spelling; usually written in kana.
Katakana spelling, sometimes used for emphasis or in informal contexts.
Kanji spelling using 無い; rare and not commonly used.
Compound of ない (nai, 'not exist') and 乳 (chichi, 'breasts'), literally 'no breasts'. The exact origin is uncertain, but it likely arose as internet slang or colloquial expression.