Badly means in a poor, wrong, or harmful way. It can also mean very much, as in “I badly want to go,” especially in informal speech.
Formed from the adjective “bad” plus the adverb ending “-ly.” It developed regular adverb uses to describe how actions are done.
English normally uses “badly” as the adverb, but in feelings we often switch back to “bad” (“I feel bad,” not “badly”). That little grammar twist shows how we treat emotions more like states than actions.
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