Definition of powerful

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pow·er·ful    (pour-fl) KEY 

ADJECTIVE:

1. Having or capable of exerting power.
2. Effective or potent: a powerful drug.
3. Chiefly Upper Southern U.S. Great: "[Everybody had] a powerful lot to say about faith and good works and free grace and preforeordestination, and I don't know what all" (Mark Twain).

ADVERB:
Chiefly Upper Southern U.S.

Very: It was powerful humid.

OTHER FORMS:
power·ful·ly(Adverb), power·ful·ness(Noun)
Regional Note:
In the Upper Southern United States the words powerful and mighty are intensives used frequently in the same way as very: Your boy's grown powerful big. The new baby is mighty purty. Powerful is used as an adjective in some expressions: The storm did a powerful lot of harm. In the same dialect region the noun power has, in addition to its standard meaning, the sense of "a large number or amount." This sense appears in the Oxford English Dictionary as common in dialectal British English of the 18th and 19th centuries: "It has done a power of work" (Charles Dickens). All these derivative senses of power and might take advantage of the notion of strength inherent in these nouns, making them natural intensives. Colloquial English is always on the lookout for ways to make language more vivid with new intensives. We think of the Upper Southern part of the United States as linguistically conservative, but in fact it has preserved uses of power, powerful, and mighty that were innovative in their time.