The Mendoza Line is baseball jargon for a .200 batting average, the supposed threshold for offensive futility at the Major League level. It derives from light-hitting shortstop Mario Mendoza, who failed to reach .200 five times in his nine big league seasons. When a position player's batting average falls below .200, the player is said to be "below the Mendoza Line".
Mendoza was a lightly-used shortstop from Chihuahua, Mexico who played for three franchises during a nine season Major League career. While his fielding was highly regarded, his hitting was not. His batting average was between .180 and .199 in five seasons out of nine.
When he had trouble staying above .200 in 1979 teammates began to chide him. "...Tom Paciorek and Bruce Bochte used it to make fun of me," Mendoza said in 2010. "Then they were giving George Brett a hard time because he had a slow start that year, so they told him, 'Hey, man, you're going to sink down below the Mendoza Line if you're not careful.' And then Brett mentioned it to Chris Berman from ESPN, and eventually it spread and became a part of the game."
Berman deflects credit back to Brett in popularizing the term. "Mario Mendoza? It’s all George Brett," Berman said. "We used it all the time in those 1980s SportsCenters. It was just a humorous way to describe how someone was hitting."
Mendoza ended up finishing 1979 below his own "line", at .198. His hitting improved modestly in 1980 and 1981, enough that even with another sub-.200 in his final season of 1982 he was able to raise his career batting average to .215.
Another term used in baseball to indicate that a hitter is hitting below the Mendoza Line is that he is "on the interstate". It derives from the syntax of the U.S. Interstate Highway System, which begins with the abbreviation "I" for "Interstate", followed by two numerals for major routes - such as "I-95" and "I-80", expressions which superficially resemble sub-.200 batting averages.
The term is also used outside of baseball to convey a similar connotation of unacceptably subpar performance: