Green’s Dictionary of Slang

1. (US, orig. milit.) a complete reversal of plans, thoughts, action.

1956		W.A. Heflin US Air Force Dict. 357: One-eighty, [...] 2. A reversal of stand, policy, or the like, as in ‘he did a one-eighty and agreed with the Commander’.
1980		G. Liddy Will 260: ‘Gordon, a one-eighty on a thing like leaving the country in forty-five minutes doesn't exactly inspire confidence’.
1998		L. Stringer Grand Central Winter (1999) 172: I still had a wall to crash into before I really got ready to do a one-eighty.

2. a 180° turn in an automobile.

1995		G.A. Haywood ‘And Pray Nobody Sees You’ in Woods Spooks, Spies and Private Eyes (1996) 179: Brother driving a blue Chrysler did a one-eighty in the middle of Wilmington Avenue.
2010		Codella and Bennett Alphaville (2011) 273: He settled for doing a one-eighty into oncoming traffic. He died instantly.