OSCAR
n. countable B2 Upper Intermediate US //ˈɔskɝ// os·car
n. a person who is always in a bad mood or complains a lot. You use this to describe someone who is acting like a grouch.
n. a person characterized by a habitually grumpy, irritable, or pessimistic disposition. Derived from the Sesame Street character Oscar the Grouch; informal in register.
Don't be such an Oscar just because it's raining.
He is a total Oscar in the mornings until he has had at least two cups of coffee.
While usually the life of the party, he becomes a bit of an Oscar when forced to discuss his taxes or any other administrative burden.
Usage
Often used in the construction 'to be an Oscar' or 'to be such an Oscar'.