To make someone feel angry, disappointed, or resentful about something, or to make a situation more unpleasant.
From the prefix 'em-' (cause to be) combined with 'bitter,' derived from Old English 'biter' meaning harsh or severe in taste. The word evolved to mean causing emotional harshness rather than physical bitterness.
Notice how 'embitter' takes a taste sensation (bitter) and turns it into an emotion—our brains are constantly borrowing from our senses to describe feelings, which is why anger 'burns' and sadness is 'dark.' Language is full of these sensory-to-emotion shortcuts.
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