AHD4 does have, for shorten : 4. To add shortening to (dough) so as to make flaky. but that's kind of circular. I'd say that's an interesting question and I'm glad you asked it.
Actually it's not circular. A crust which is "short" is flaky, light and tender. You shorten a pastry dough by adding the right form of grease to make it short and by not overworking it so that less gluten is made. Shortening is the material you add to make the pastry short.
Mom's piecrusts will always be shorter than mine but then she's shorter than I am too so fair's fair.