Contrary to it's name, the Boston Butt does not come from the rear end of a pig. It is actually the upper part of the shoulder. So how did it get it's name? While there is some historical debate over this, it seems to be that the city of Boston became know for this particular cut of pork shoulder (similar to New York Strip Steaks or St. Louis style ribs) and somehow the name stuck. The "Butt" in this case is referring to the larger or thicker end of something (rifle butt, cigarette butt); the end of the shoulder. It also makes for some funny t-shirts and slogans!
Picture from: https://br.pinterest.com/pin/i-like-pig-butts-and-i-cannot-lie-you-other-grillers-cant-deny-grilling-bbq-barbecue-pork-lacajachi--232287293263700944/