I found a grand total of 15 results, almost of of which came from the site where I first found the phrase. However, none of them told me what exactly it was. So please, if anyone knows anything about rogue's slang (from the 18th century or any other point in time), please, please let me know!
ETA: My original search was also depressingly unsuccessful. Can anyone suggest an 18th-century euphemism for 'penis' that a fairly crude young woman might use when talking to a friend?
ETA #2: I think I have my first question answered. Thanks, folks!
ETA #3: Second question pretty exhaustively answered. Thanks, everyone- you've been a great help!