Acronym used to describe a person who you are not sure if its a man, a women, or a unknown.
She+HE+It = Shit
Or a acronym used to describe someone who has had multiple or is going threw a sex change(s).
Example: Did you see that S.H.I.T. that just crossed the street.
Your walking down the street and you see what you think is a chick but after closer inspection you relize it might be a dude. so then you put that person into the it catogory. So you use the acroynm S.H.I.T. to describe that person.
In the second case you have seen or know somebody who has or is going threw sex operation(s). So they are neather a man or a women there a cobination of a She, He, It.