Green’s Dictionary of Slang

(US) a fig. place representing eccentricity or anything outside the norm; usu. as from leftfield/out of leftfield.

1949		A. Shaw ‘Vocabulary of Tin-Pan Alley’ in Notes VII:1 34: The pluggers are responsible for the argot of the business. They contribute such colorful words and expressions as:[...] out of left field.
1952		W.R. Burnett Vanity Row 54: ‘We got a few facts. But we’re still out in leftfield. Why would she kill him?’.
1960		Mad mag. June 46: Out of left field you come hacking.
1961		‘Spoken Lang. of Medicine’ AS XXXVI:2 147: out in left field, v.phr. disoriented, out of contact with reality.
1970		Time 9 Mar. in Barnhart Dict. of New Eng. 256: An increasing number of candidates are emerging from leftfield to give voters surprising options.
1983		Orange Coast mag. July 21: The notion is from leftfield, if it exists at all, that Heather is a bit cool, perhaps aloof.
1990		T. Pluto Loose Balls 53: The guy later became the manager of the Monkees, and he was sort of far out in left field to be in pro basketball.
2004		D. Valentine Strength of the Wolf 453: They cite me for leaving the office without an escort, and for letting dope leave the country. Then they come at me from leftfield; they mention Gohde and Dolce.
2008		Guardian 15 Oct. 🌐 Did anyone expect The White Tiger to win? Certainly, it struck me as a choice that came from way out of leftfield.