"Plonk down" is a commonly used phrasal verb that means to place something heavily or unceremoniously. However, there are many other ways to express this action using synonyms. For example, you could use the words "plop," "drop," "deposit," "set," "place," or "lay" to describe the action of putting something down. Depending on the context, you may want to choose a different word that conveys a specific tone. For instance, "plop" could indicate a more casual or playful action, while "deposit" might be interpreted as more formal or business-like. Ultimately, the choice of synonym depends on the desired effect the speaker or writer wants to achieve.