To put a biscuit, piece of bread, etc. into a liquid such as tea, coffee, or soup for a short time before eating it. (Cambridge Dictionary)
To put something into liquid for a short time. (Cambridge Dictionary)
To dip (something, such as a piece of bread) into a beverage while eating. (Merriam – Webster)
If you dunk something such as a biscuit or a piece of bread in a drink or in soup, you dip it into the drink or soup before eating it. (Collins Dictionary)
Dip (bread or other food) into a drink or soup before eating it. (Oxford Dictionaries)
Dunking (biscuit). Wikipedia.
Slam dunk. Wikipedia.