I always wondered if it came from the horse-and-buggy days -- a 'beater'
being a horse that you would whip mercilessly because you didn't care
about it.

I have no idea if this is true, though.  I've never seen a formal
etymology.
             _ _
  __ _ _   _| | |  | David M. Brodbeck (N8SRE)              Ypsilanti, MI
 / _` | | | | | |  +-----------------------------------------------------
| (_| | |_| | | | @ cyberspace.org
 \__, |\__,_|_|_| "I thought it was another false start in the never-
 |___/             ending, weird hope that within our lifetime, we could
                   actually make a robot that could potentially turn on
                   us humans and kill us.  Mankind will have accomplished
                   something when that day comes."  -- John C. Dvorak