Definition of fill in

The phrase "fill in" is a phrasal verb that has originated from English. In its earliest known use, dating back to the 16th century, "fill" was utilized independently as a transitive verb, implying "to make full," "to complete," or "to supply." Later, around the 17th century, the expression "fill up" emerged, conveying the same meaning in a more explicit manner. However, the use of "fill in" as a phrasal verb did not come into being until many centuries later, around the mid-1800s. It may have initially been used in the context of military training drills, to indicate that a candidate was required to fill a specific role or afford a particular responsibility within an organized assembly. In these drills, soldiers were asked to "fill in" when someone dropped out or was unable to perform their duties, thus completing the unit's composition. The expression soon spread beyond the military context and now signifies taking the place of someone or something that is missing or incomplete, quite comprehensively. This phrasal verb is now common in various aspects of everyday life, from filling in a form to filling in the missing jigsaw puzzle pieces, or even filling in light musical breaks in a performance.