論外

Etymology
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Probably from compound.

Alternately, may be a Japanese coinage from 🇨🇬-derived elements, as a compound of.

Adjective

 * 1) irrelevant, beside the point, out of the question

Usage notes
Listed explicitly in some dictionaries  as a -na type adjective. Also used as a noun followed by particle to modify other nouns.

Noun

 * 1) something outside the scope of discussion, something that is off-topic