A person who abases or humiliates someone else; someone who treats others disrespectfully or reduces their status.
From 'abase' (verb) + '-er' (agent noun suffix), creating a noun for the person performing the action of abasement.
English uses '-er' to turn verbs into people who do things—an abaser is specifically a person in the business of humiliating others, which sounds like a pretty terrible job!
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