for the most part
"he is mainly interested in butterflies"
Forcefully, vigorously.
"She mainly attacked his arguments until he finally conceded the point."
In plain English: Mainly means mostly or for the most part.
"The crowd was mainly made up of local residents."
Usage: Use mainly to mean primarily or mostly when indicating that something happens in most cases but not all. Do not use it to describe forceful or vigorous actions, as that is not its common everyday meaning.
The word mainly comes from Middle English, where it was formed by adding the suffix "-ly" to "main." It originally meant "chiefly" or "for the most part," a meaning that has remained consistent in modern English.