Slate Article: Why Are Guys Afraid to Wear Speedos?

[Byron]

Yes - never could understand why important parts of the male anatomy should be unwanted rubbish to go in the trash can.
Plenty of examples of usage to be found but no indication of this particular derivation (it being also heroin and a chinese boat of course).
Middle English jonk had a particular nautical definition - it was old rope or cable being reused on a ship - but again something worn out and useless for original purpose and so inappropriate for its frequent use now in the anatomical sense.