Mainly or primarily; for the most part or above all other considerations.
From chief (from Old French chef, meaning 'head') plus the adverbial suffix -ly. The word evolved from the concept of leadership hierarchy to express priority or predominance in abstract contexts.
This word retains the essence of its 'chief' root - just as a chief leads a tribe, 'chiefly' signals which reason, cause, or factor leads all others. It's a linguistic democracy where one idea gets elected as the primary representative!
Complete word intelligence in one call. Free tier — 50 lookups/day.