Tab- How did you guys come up with his name? It's really unique!
LOL! Believe me, it was a bit of an ordeal!!
We (mostly *I*) made a list of names we liked. Then I started asking advice on here, including wondering if anyone knew if there a was a particular sound in names that made them easier to learn. Someone mentioned that their trainer said that one-syllable names might be easier, so I took that into consideration.
As far as where it originally came from...I'm a HUGE music fan. Some of the names on our list were Manson, Danzig, Ozzy...I guess you can tell what sort of stuff we like.

Anyway, Cash is actually from Johnny Cash, who I'm an ENROMOUS fan of. DH and I are both fans - me since I was a kid (I grew up on country music) and him in the last few years - ever since Johnny Cash did a NIN song "his way".
So...one syllable name, plus The Man In Black...and you have Cash!
