Grump dumpling
Someone who never has anything nice to say, ever. There's always a rude comment coming out of their mouth.
Person 1: Becca was so rude today. She pretty much told me I suck at my job.
Person 2: Don't pay any attention to her. She's just a grump dumpling.
Person 2: Don't pay any attention to her. She's just a grump dumpling.