Restless and eager to move or act; fidgety and impatient.
American slang from the 1960s, from the phrase 'have ants in one's pants,' which dates to the 1930s. The imagery comes from the uncomfortable, fidgety feeling one would have with ants crawling inside clothing.
The evolution from 'ants in your pants' to 'antsy' shows how vivid metaphors become streamlined into single words. The image is so perfect - ants are constantly moving, and having them in your clothing would make anyone fidgety and restless.
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