IPA: /ˈbrɪm.fʊl/
KK: /brɪmˈfʊl/
Filled completely, so that it is almost overflowing.
The cup was brimful of hot coffee, ready to spill if not handled carefully.
Brimful is formed from "brim" (from Old English "brymm", meaning edge or border) and "-ful" (meaning full of). The word "brimful" means full to the edge or overflowing.
Think of something that is full to the 'edge' ('brim') — that's why 'brimful' means overflowing.