In a way that shows deep anger, regret, or sadness; or with an intensely sharp, unpleasant taste.
From 'bitter,' which comes from Old English 'biter' related to 'bite,' suggesting something sharp or cutting. The emotional meaning developed metaphorically from the harsh taste sensation.
Your tongue has 'bitter' taste receptors that evolved specifically to detect poisons—so saying something is 'bitterly disappointing' taps into your brain's ancient poison-detection system!
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