I suspect the point of the assignment is to use hashing.
Given that as the case:
If crash hashes to the same value as crashes, use that (the hash) as your key and the meaning as your value. Hash crash and crashes from one thing that you know they share: the meaning of the
word.
A little bit of circular reasoning but if the only thing this accomplishes is to get you thinking that's at least a step in the right direction.
And while you're overriding hashCode(),
you should probably override equals() as well...just as a complete side note.