1940s-50s upper midwestern slang for penis.
Joe and Mike were peeing off opposite sides of a bridge. Mike wanted to brag about the size of his jinker, and said to Joe, "This water sure is cold." Joe, not wanting his jinker to be underestimated, replied, "Yeah, and deep."
Also written as ---30---, -30-, /30/ or ###.
Typography and news media shorthand meaning "end of story."
And they all lived happily ever after. The End.