For many American speakers of English, the phrase “ants in your pants” is so familiar that its meaning has never been explicitly questioned. In fact, the phrase is an idiom, or an expression that does not make much sense when its individual parts are examined, and therefore it may bewilder non-native English speakers, or even non-American English speakers. Essentially, it's used to describe a person who is fidgety and restless, often because he is distracted by some pressing thought. The exact origin of this idiom is unclear.
“Ants in your pants” is an idiom, so it would be difficult or even impossible for a person unfamiliar with a particular idiom to decipher its meaning when it is presented in isolation, without any contextual information. Other examples of English-language idioms include “play by ear,” “spill the beans,” or “on your high horse.”
This phrase is used to describe an individual who is fidgety and restless, often because he is distracted or anxious. For instance, a kindergarten student who is looking forward to a birthday party after school might have difficulty sitting still during class. This student may be said to have ants in his pants, although obviously, the student does not literally have these insects in his clothing. Rather, his fidgety movements are visually similar to the movements that might be displayed by a person whose clothes are filled with bugs.
It is not known when the phrase “ants in your pants” was first introduced, or who initiated its usage. Research shows, however, that the phrase has been widely used since the mid-20th century, and was probably introduced even earlier. Additionally, it is largely considered to be an Americanism, and as such, it may not be familiar to English speakers from countries such as Ireland, England, South Africa, or Australia.