In a manner that is extremely offensive, hateful, or deserving strong condemnation and disgust.
From 'abominable' plus the standard adverb suffix '-ly'. The entire word chain traces back to Latin 'abominari' (to turn away in disgust, to abhor).
When people say 'abominably' today, they're often exaggerating for dramatic effect—it's Victorian language for 'really, really badly', which is why you'll hear it in fantasy novels and period dramas rather than everyday speech.
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