Sentence:  <WSD>Nicking</WSD> a horse has been generally believed to be attended with much difficulty, and to require great ingenuity and art to perform the operation. The <WSD>nicking</WSD> alone, is by far the easiest part, as the curing and pullying requires considerable attention and trouble. Nicking is an operation performed for the purpose of making a horse carry an elegant artificial tail, which adds much to his beauty and value. 
Meaning of the word marked with <WSD> tags:  To make a crosscut or cuts on the underside of (the tail of a horse, in order to make the animal carry it higher).