Wikipedia says it's the military slang name for scavenging birds of prey & vultures - originally the BLACK KITE or Kite Hawk met by British troops in the 'Middle East'.
"At the transit camp the British soldier normally made his acquaintance with the kite-hawk [i.e. black kite], known familiarly as the 'shite-hawk'. 'There used to be thousands of them,' remembers Charles Wright. 'When one drew one's food from the cook-house and went to take it across to the dining room to eat at the tables underneath the sheds, these kite-hawks would swoop down and take the lot off your plate if you weren't careful. So you had to walk waving your arms above the plate until you got it under cover.' "
— Charles Allen, Plain Tales from the Raj
The term "shitehawk" was also used as a derogatory term for an offensive or unpleasant person, equivalent to the word "shit-head".
While thinking about toileting(!) - in the late '50's & early '60's me as a young white boy working with some immigrants from Saint Kitts, Caribbean islands - I got to know a few of the guys quiet well. And I miss that side of London life more than any other aspect - that Jamaican music, reggae & 'ska' is great stuff ..
- Sitting on the underground train in the rearmost carriage- homeward bound, late night London - the young Jamaican Guard closing the train's doors at each stop before signalling it clear to pull-out -
" MIND THE DOORS YOU RAAS-CLAATS" - He saw me (white boy) laughing with him and we exchanged grins ...