na-adjective, noun
hardcore enthusiast; fanatical; geeky; specialized
Used as a noun for an intense enthusiast and especially as a na-adjective for people, tastes, questions, information, etc. that are very specialized or appeal to enthusiasts. It often has a casual, slightly amused tone rather than the severe English sense of 'maniac'.
彼の音楽の趣味はかなりマニアックだ。
His taste in music is quite specialized.
マニアックな質問にも店員
The clerk answered even the geeky questions carefully.
彼はかなりのマニアックで、古いレンズの型番まで覚えている。
He is quite a hardcore enthusiast and even remembers the model numbers of old lenses.