How to say "illegitimate" in Japanese

• 妾腹しょうふく
  nounIllegitimate in the sense of born to a mistress; it describes a child's birth, not improper actions or claims. Being less common, it is seldom the word you need outside that narrow sense.
• 妾出しょうしゅつ
  nounAlso illegitimate in the sense of born to a mistress, essentially the same as 妾腹. Less common, it is rarely the word you need, and not the word for the abstract sense of something improper.
• 不適正ふてきせい
  na-adjectiveIllegitimate in the sense of improper or unacceptable, as of conduct judged not proper; it is less common and has nothing to do with birth status, so it never describes a child.

For illegitimate meaning born to a mistress, 妾腹 or 妾出 (both less common); for illegitimate meaning improper or unacceptable, use 不適正, which is also less common.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

• 妾腹しょうふくillegitimate (e.g. born to a mistress)
• 妾出しょうしゅつillegitimate (e.g. born to a mistress)
• 不適正ふてきせいimproper