barn [OE] A barn was originally a place for storing barley [OE], the word coming from Old English from bere ‘barley’ and ern ‘house’. In the 1940s barn started to be used in particle physics as a unit of areas. It is apparently from the phrase as big as a barn door, a long-established measure of size. In the mid 19th century barnstormers were travelling actors who put on plays in country barns. The word was transferred to people who put on flying displays in the 1920s.