n'at is an integral part of the yinz dialect of english as spoken by residents of the greater Pittsburgh area. Its usage is similar to the Canadian
" in that it is found to end just about every other sentence spoken by a native yinzer
According to local mythology, "n'at" is a contraction of the words "and" and "that". It is supposedly used to imply that there is more included in the sentiment of the speakers statement than was directly implied. For instance the phrase "apples and oranges n'at" would logically refer to not just apples and oranges, but also to many other appropriate fruit.
Due to the excessive usage of this phrase however, it has apparantly lost its original meaning and now has become nothing more than an overused sound marking the end of a sentence.