A person or thing that causes bitterness or resentment in others.
From 'embitter' with the suffix '-er' added to indicate an agent or person who performs the action; this creates a noun meaning 'one who embitters.'
This is a rare word because we usually don't need it—we'd say 'someone who causes bitterness' instead—but it's a perfect example of how English allows you to build new words by adding suffixes, like a linguistic toolbox you can use to create almost any word you need.
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