In a way that is extremely dislikable, hateful, or worthy of strong disapproval and disgust.
From 'detestable' (worthy of hatred) plus the adverb suffix '-ly.' From Latin 'detestari' (to curse or abhor).
You might say a villain in a story acts 'detestably'—it means their behavior is so bad it makes viewers feel genuine hatred, not just mild dislike!
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