From BR Bullpen
A term used to refer to players who have a batting average under .200. The term was derived from light-hitting shortstop Mario Mendoza, though Mendoza's career average was .215. The term's origins have often been credited to George Brett, who used it in 1980. Mendoza claims it was first used by Tom Paciorek in 1979, but Paciorek says that Bruce Bochte employed it first. Either way, it was started by a teammate of Mendoza on the 1979 Mariners.
The Mason Dixon Line, a variant of the Mendoza line, has been proposed as a better standard for poor batting average, as it is closer to .200.