主に
おもに · omoni · JLPT N3 · adverb
mainly
主に = 主 (main, master) + に (adverb ending): in the main way
主 means main or master, as in 主人 (master of the house). Adding に makes it an adverb, so 主に means mainly. A sound hook: omoni sounds like oh, money, as in it is mainly about money.
主 — flame/lamp on top of a stand 王
Originally a pictograph of a lamp with a flame on top of its stand, representing the lord or master of the household who tended the fire. The meaning extended from 'lamp/host' to 'master, owner, main, principal'. The character 炷 (wick) preserves the original 'lamp' sense.