What is -er?
The addition of the suffix -ER to an adjective, to create a noun. Thereby turning a characteristic or behaviour into a descriptive name. If someone behaves in a gay way, they become a 'gayer' or if they're being greedy they are a 'fatter'.
Stop picking on me man. You're such a mean-ER.
You'll never get laid if you keep being such a stupider