Definition: soft slushy mud or snow
Sposh is thought to be a combination of the words slush and posh, but before you get overly excited about the new factoid with which you plan on boring your in-laws to tears, there are a couple of things you should know. First, posh does not itself come from "port-outward-starboard-home," or any other acronym. Second, the posh that sposh is based on is a totally different kind of than the one meaning "fashionable"; this one is an archaic word, meaning “a slushy mass (as of mud or broken ice).”
I enjoy not less what may be called the negative side of spring, — those dark dank, dissolving days, yellow sposh and mud and water everywhere, — yet who can stay long indoors?
—John Burroughs, Birds and Poets, 1895