Chapman may be the only source who thinks it could have come from a mispronounciation of haphazard.
Look at Lighter’s cite from 1863, “there goes our half-assed Adjutant,” and suggest where haphazard comes into play in the meaning of the word. Yes, I know that the adjective meant two different things in the mid/late 1800’s, but the phrase was already establised by 1863 and couldn’t have come from haphazard, IMHO.