1. To fling or plop oneself down onto some surface or thing. I knocked a few pillows onto the floor when I flopped down on the couch. As soon as Paulina flopped down onto the bed with a big sigh, I knew that her date must not have gone well.

2. To plop something down onto some surface or thing. In this usage, a noun or pronoun can be used between "flop" and "down." Once I realized she was angry with me, I flopped her Valentine's present down and left.