IPA: /ˈɡreɪtˌkoʊt/
KK: /ˈɡreɪtˌkoʊt/
A thick and warm coat that is usually worn over other clothing, especially in cold weather.
He wore a greatcoat to keep warm during the winter.
Greatcoat → The word is formed from "great" (from Old English *grēat*, meaning large or big) and "coat" (from Old French *cote*, meaning a garment). A greatcoat is a large, heavy coat typically worn for warmth.
Think of a 'large' ('great') garment ('coat') that keeps you warm — that's what a greatcoat is.