Part of speech: Adverb
for the most part
Example: he is mainly interested in butterflies
In the main, Primarily, Principally, Chiefly, Mainly
The word "mainly" comes from Middle English maynly.
main – A related word in the same word family.
ly – A related word in the same word family.
The printing press standardized Middle English spelling, freezing many irregular forms we use today.