Sentence:  Cows were kept in backyard barns , boys were hired to drive them to and from the pasture on the edge of town , and familiar to the ear , morning and evening , were the boys ' coaxing voices , the thud of hooves , and the <WSD>thwack</WSD> of a stick on cowhide . 
Meaning of the word marked with <WSD> tags:  a hard blow with a flat object