https://www.urbandictionary.com/define.php?term=Doomer&defid=14451945

A lack of faith in humanity, or people in general.

Giving up on all humans, or groups of people, for perceived slights or injustices committed by a few.

Lacking confidence in the future of humans because of an event, directly affecting their life or not, despite any positive response that happened.

Note: This is not limited to any particular political group, affiliation or generation.

Doomer: I just lost faith in humanity because someone hit a child walking down the street and just drove off. Nevermind the dozen people that stopped to check on them, and that called the police while doing everything they could to help the person in need. That one human who hit them makes everyone bad in my eyes.

Normal people: Ok doomer.