SIMPLE! Schlong is NYC Yiddish. Where do you think Donald Trump picked this up from?? Some UFO visitor or William Zeckendorf?
https://en.wikipedia.org/wiki/William_Zeckendorf
I agree it was in the vernacular. My point is it got there mistakenly—what William Safire used to called a back-formation. He would have had the definitive word on this subject.
It could also be Brooklynese or some other form of local slang...I'll wager even Trump himself doesn't know its exact origins.