Green’s Dictionary of Slang

(US) unorthodox.

1989		Stereo Review LIV 133/2: So unconventional is his vocal quality and so left-field are a number of his songs [etc.].
1999		Indep. Rev. 12 Aug. 8: The kind of left-field philosophical musing to which Izzard is given.
2000		Indep. on Sun. Reality 7 May 18: We have always been as left-field as possible.
2002		H. Ong Fixer Chao 367: This lie was so left field it left me scratching my head for days.
2015		I. Welsh Decent Ride 21: His totally left-field attack with a sharpened screwdriver on Evan Barksdale.