Dipping something in and out of a liquid to soak it. A good example is a dunking stool. Dunking is a very good thing to do with a Rich tea biscuit and a cup of tea. You dip it in until it nearly snaps with soggyness then plonk it in your gob. Lovely.

Dunking is also possible in basketball: you jump so high that you can "put" the ball into the basket instead of throwing it in. Very popular in NBA.

See also: Michael Jordan.