- Very difficult or impossible to forget
- The things he subjected me to were horrible and unforgettable.
- 1961 July, J. Geoffrey Todd, “Impressions of railroading in the United States: Part Two”, in Trains Illustrated, page 419:
- 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.
which is very difficult to forget