Sentence:  & thenne he rode after the bore / & thenne syre laūcelot was ware where the bore set his <WSD>ars</WSD> to a tree by an hermytage / Thenne sir launcelot ranne atte bore with his spere / & ther with the bore torned hym nemly 
Meaning of the word marked with <WSD> tags:  The buttocks or more specifically, the anus.