A person who dislikes something or someone; one who has aversion to or disapproves of something.
From 'dislike' + '-er' suffix (from Old English '-ere', denoting an agent or one who performs an action). The '-er' creates an agent noun.
The '-er' suffix is one of the most common ways to name someone who does something—'teacher', 'runner', 'player'—so 'disliker' is logically formed but rarely used because English prefers saying 'someone who dislikes' instead.
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