Sentence:  Half crazed by the weight dragging , the <WSD>dust</WSD> , and the heat , the horse leaped our wall , dashing out the rider 's brains against it , and leaving him lying there among us - while the horse crashed away through the brush . 
Meaning of the word marked with <WSD> tags:  fine powdery material such as dry earth or pollen that can be blown about in the air