Refers to someone who shows off their knowledge or is overly absorbed in books and learning, often with a negative or mocking tone. The word is relatively rare and literary.
He is a pedant who always wants to use difficult words.
That professor is said to be a pedant, but in fact he has deep knowledge.
Compound of 衒 (gen, 'to show off, display') + 学 (gaku, 'learning') + 者 (sha, 'person'). The exact historical derivation is uncertain, but the word likely entered Japanese from classical Chinese texts.