Someone judging a person that they do not know and with little to no information, giving them a derogatory label as a result of their bias. Thus, drawing a conclusion about the kind of person they are based on said label.
“That guy legit said I must be a slut if I drink tequila shots. He obviously judge labeled us from across the bar”
This is something most people should learn. Stop judging and saying what people are and aren't if you don't know them, if you don't live their lives. Only them know the reality.