unforgettable

adj

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.”

   “The leading engine was one of the Class Y6 2-8-8-2 compound articulateds, [...] The stack noise of one of these great brutes slogging up a grade was quite unforgettable.”