A rotter is a person who treats other people badly — selfish, unkind, not to be trusted. It is British, and it sounds a little old-fashioned or playful now, the sort of word you meet in older stories or when someone jokes in a posh voice. You might call a man a rotter if he lies, cheats, or takes what is not his and does not care. It is not a heavy swear word, but it is not polite either; it is a sharp, almost comic insult. Americans almost never say it. Underneath is the idea that the person is “rotten” inside. You will hear it more in British books and films than in everyday talk on the street.

壞蛋、卑鄙的人（偏英式、有點老派，帶一點戲謔）。