Can someone please explain how "lampost" is an acceptable abbreviation for "lamp post"?

I questioned this on Facebook after seeing the strange-looking "lampost" there. Is it a post for a lam, or an ost for a lamp? Remember, if there is only one "p" it can only belong to "lamp" OR "post", one or the other. Not both. So what is a lampost? Is it a post for a lam, or an ost for a lamp? LOGICAL ANSWERS ONLY, PLEASE.