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A barn (symbol: b) is a metric unit of area equal to 10-28 m2 (100 fm2). Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is also used in all fields of high-energy physics to express the cross sections of any scattering process, and is best understood as a measure of the probability of interaction between small particles. A barn is approximately the cross-sectional area of a uranium nucleus....
Etymology
During Manhattan Project research on the atomic bomb during World War II, American physicists at Purdue University needed a secretive unit to describe the approximate cross-sectional area presented by the typical nucleus (10-28 m2) and decided on "barn". They considered this a large target for particle accelerators that needed to have direct strikes on nuclei, and the American idiom "couldn't hit the broad side of a barn" refers to someone whose aim is very bad.[2] Initially they hoped the name would obscure any reference to the study of nuclear structure; eventually, the word became a standard unit in nuclear and particle physics.[3][4]
YOu all know where the word "tank" came from, right? The motorized cannon.