A morose, melancholic person; pessimist; crape-hanger
[1940s+; as the name of a comic-strip character, Gloomy Gus is found by 1904]
I shall tell it to gloomy gus when she and I grow to be bosom friends.
"Mrs. Lee invited you, Brig," reminded gloomy gus, who believed that every man should do his duty.
He carried a little water for gloomy gus, but stubbornly refused the job of flunky.