To dunk is to dip biscuit, bread, cake, or doughnut into a beverage, especially tea, coffee, or cold milk as in the popular American snack milk and cookies. Dunking releases more flavour from confections by dissolving the sugars, while also softening their texture.
A popular form of dunking in Australia is the "Tim Tam Slam", also known as "tea sucking". The physics of dunking is driven by the porosity of the biscuit and the surface tension of the beverage. A biscuit is porous and, when dunked, capillary action draws the liquid into the interstices between the crumbs.
Dunking is also used as a slang term for intinction: the Eucharistic practice of partly dipping the consecrated bread, or host, into the consecrated wine, either by the officiant before distributing it or by the communicant before receiving it.