The word "mainly" is an adverb that means primarily or chiefly. It is used to indicate the most important aspect of something or to suggest that something is true most of the time, but not always. For example, when you say "I eat mainly vegetables," it implies that vegetables are the primary component of your diet.
The word "mainly" originates from the word "main," which comes from the Latin "magnus," meaning "great." The suffix "-ly" turns the adjective into an adverb, indicating the manner in which something is done.