To fill (something) to overflowing.(verb-transitive)
To become too full.(verb-intransitive)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "overfill" in a sentence
"They allow for almost no "overfill," i.e., if one small thing (say a strawberry) is taller than the edge, getting the lid to seal is nearly impossible."
"The more you pre-wash, the less detergent you need, but never overfill to try and compensate for not pre-washing."
"After nearly running out of gas between Shasta and Susanville (because I didn't believe there could really be that many miles between gas stations) it was totally awesome to overfill the tank when I finally found some fuel."