To set someone or something upright against or atop something else in a haphazard or careless manner. Someone plonked the sack of money up against the side of the building and just left it there! The odd little café was just sort of plonked up on top of the little hill all on its own. He plonked the little girl up on the his shoulders so she could see the parade.

Set upright against or atop something in a haphazard or careless manner. The odd little café was just sort of plonked up on top of the little hill all on its own. We discovered the sack of money plonked up against the side of the building, without a soul in sight!