In a big manner; greatly or to a large extent; at a large scale.
From 'big' plus the adverbial suffix '-ly.' Though somewhat archaic, the word has seen renewed colloquial usage in modern English.
The word 'bigly' became famous in the 2010s through political discourse, making linguists laugh because '-ly' adverbs from adjectives are supposed to make words feel more formal, but 'bigly' somehow sounds more casual and emphatic.
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