shortening n.

1540s, "action of making short," verbal noun from shorten.

The meaning "butter or other fat or oil used in baking" (by 1796) is from shorten in the cookery sense "make crumbly," attested by 1733. This is from short (adj.) in a secondary sense in reference to food, "friable, easily crumbled," attested by early 15c. in cookery books.

Hence short pastry, in full shortcrust pastry, that to which lard or butter has been added to make it soft and flaky. This also is the short in shortbread and shortcake. Also shortening bread (by 1884), a U.S. Southern specialty.

Short (adj.) as "easily crumbled" also has been applied to things other than food, and this use of it perhaps implies "having short fibers," as materials with short fibers fall apart more easily (e.g. short-staple cotton).