Definition of "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.

1903 July, Jack London, “The Law of Club and Fang”, in The Call of the Wild, New York, N.Y.: The Macmillan Company; London: Macmillan & Co., page 44

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.

1961 July, J. Geoffrey Todd, “Impressions of railroading in the United States: Part Two”, in Trains Illustrated, page 419