it is possible that "schlong" has derived from the german word "Schlange" (meaning "snake"). Though that word is not pronounced like "shlong", it sounds more like "shlung-eh" or "shlahng-eh". So there has to be a link in between the german and the english - i thinks its a yiddish word, "schlong", still meaning "snake".
Schlong-along, my sweet valentine!
(a person is to told to be more qick like a snake)