To overfill means to put too much of something into a container, filling it beyond its proper capacity. When you overfill a glass with milk, it spills over the rim onto the table. When someone overfills a backpack, the zipper won't close and books stick out the top.

The word combines over (too much) and fill (to make full). You can overfill anything that has limits: a balloon until it pops, a swimming pool until water floods the deck, or a schedule with so many activities that there's no time to rest. A baker who overfills cupcake tins creates a mess in the oven when batter spills out during baking.

People use the word figuratively too. A teacher might warn against overfilling your essay with unnecessary details that distract from the main point. Someone who tries to overfill their day with plans might end up exhausted and unable to enjoy any of them properly.

The key idea is exceeding the right amount. Filling something completely is often good, but overfilling creates problems: spills, waste, broken containers, or things that don't work as intended.