1. Be extremely restless, uneasy, impatient, or anxious, as in This child just can't sit still; she must have ants in her pants. This rhyming idiom calls up a vivid image of what might cause one to be jumpy. [Slang; 1920s]
2. Be eager for sexual activity, as in Bill's got ants in his pants for Rita. This usage is less common today. [Slang; 1920s]