what does it mean to "Johnny Cash" something?

Answer 1

when you finish something. It's a reference to finishing, or "cashing" a bowl of marajuana. Since the term is "Cashing a bowl" then people turned it into Johnny Cashing something. Sort of like calling someone "Maynard" instead of "Main".

Answer 2

Have a look at the last 3 definitions in this:

http://www.urbandictionary.com/define.php?term=johnny+cash&page=2