Green’s Dictionary of Slang

(US) go away! leave me alone!

1947		‘Imaginary Diseases’ in AS XXII:4 Apr. 305/1: f-o-itis. Pronounced eff-oh-itis; meaning ‘malingering’; based on initial letters in the manner of snafu.
1970		Private Eye 6 Nov. 1: F. O. the lot of you!
1989		P. Munro Sl. U. 82: F.O.! Get out of here! You make me sick!