noun which may take the genitive case particle 'no', noun
outside one's area of expertise; outside one's field
Used to indicate that something is not within one's professional or academic specialty. Often appears in the pattern 専門外の〜 or 専門外で〜.
その質問は私の専門外です。
That question is outside my area of expertise.
専門外の分野について話すのは
It's difficult to talk about a field outside one's specialty.