IPA: /ˈoʊvərˌfʊl/
KK: /oʊvərˈfʊl/
Filled beyond capacity or limit, causing it to overflow.
The overfull cup spilled coffee all over the table.
Overfull → It is formed from "over-" (meaning excessively) and "full" (from Old English "full", meaning filled to capacity). The word "overfull" means filled to an excessive degree or beyond capacity.
Think of something that is filled "excessively" ('over-') — that's why overfull means too full.