In a way that mixes both sad and happy feelings, like feeling nostalgic about something good that's now over.
From 'bittersweet' (combining 'bitter' and 'sweet'), plus the adverbial suffix '-ly,' the word evolved to describe actions or expressions that embody this emotional mixture.
This word captures something English does beautifully—combining opposites to express complex feelings that simple words can't quite reach, which is why 'bittersweetly' appears in poetry and literature far more than in everyday speech.
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