IPA: /ˈtɒp.fʊl/
KK: /tɑpˈfʊl/
Filled completely to the highest point.
The glass was topfull, spilling water over the edge.
This word originates from Old English, where 'top' means the highest point or summit, and 'full' means filled or having a lot of something. Therefore, 'topfull' refers to being filled to the highest point or brim.
Think of something that is filled to the very 'top' — that's what 'topfull' means, being full to the highest point.