Also a non-American (Brit). Just means something unusual. According to the 2007 Concise New Partridge Dictionary of Slang and Unconventional English, the phras

Also a non-American (Brit). Just means something unusual. According to the 2007 Concise New Partridge Dictionary of Slang and Unconventional English, the phrase came from baseball terminology, referring to a play in which the ball is thrown from the area covered by the left fielder to either home plate or first base, surprising the runner. Variations include "out in left field" and simply "left field"