Definition:

killjoy; someone who takes a pessimistic view of things

About the Word:

Black crepe fabric was once an important part of mourning ritual. It was sewn into dresses and veils, wrapped in bands around hats and arms, and draped over doors.

We can speculate that to those who started using this insult, a crepehanger was a "killjoy" almost in a literal sense – the sort of person who took pleasure in a funeral.