"Pig iron" is used to describe any unguided, free fall bomb used in the American armed forces. These relatively inexpensive explosives are dropped and fall under the influence of gravity alone toward their target. Since this is the direct opposite of a smart bomb, these are often referred to as dumb bombs.
Most pig iron bombs have a fuse with a propeller on the front. The propeller is secured in place until released by the wizzo. As the bomb falls, the propeller turns as the wind drives it around. After a certain number of turns, the bomb arms itself and the fuse will detonate the bomb when it impacts another object: building, tank, ground, or any other object beneath the airplane.
Some common pig iron bombs include the Mk-84 (2000 lbs) and Mk-82 (500 lbs) Low Drag General Purpose (LDGP) bombs, the Mk-20 "Rockeye" anti-armor bomb and the BLU-82 "Daisy Cutter" which has gained notoriety due to its use in the United States military action against Afghanistan.