He plunked down a large sum on an expensive pair of shoes while window shopping.

plunked 是动词 plunk 的过去式，plunk 是拟声词，模仿物体重重落下的声音，引申为"突然放下"或"随意扔下"的意思。在这里指"随手花掉一大笔钱"的动作。