noun-modifying (irregular)
unforgettable; never to be forgotten
Literary or emotive prenominal modifier; often used in poetic or reflective contexts to describe memories, people, or experiences that cannot be forgotten.
忘れ得ぬ思い出が心に残っている。
Unforgettable memories remain in my heart.
彼は忘れ得ぬ人だ。
He is an unforgettable person.
忘れがたい is more common in modern Japanese and means 'hard to forget', while 忘れ得ぬ is a more literary, classical-sounding expression meaning 'cannot forget'.
Compound of 忘れ (the continuative form of 忘れる 'to forget') + 得ぬ (the classical negative potential form of 得る 'to be able to'), literally 'cannot forget'.