Green’s Dictionary of Slang

1. (US) honestly.

1930

R. Whitfield Green Ice (1988) 109: But I repeat – it isn’t divorce stuff. Heads up.

2. a phr. used to indicate affirmation.

2001

(con. 1964–8) J. Ellroy Cold Six Thousand 20: ‘The pro. Did he get out?’ ‘Heads up on that. He got down to McAllen and walked across the border.’.