unforgettable

adj

Etymology: From un- + forget + -able or un- + forgettable.

1. Very difficult or impossible to forget.

   “The things he subjected me to were horrible and unforgettable.”

   “He had never seen dogs fight as these wolfish creatures fought, and his first experience taught him an unforgetable lesson. It is true, it was a vicarious experience, else he would not have lived to profit by it.”