The likely
origin of this
expression has to do with
geology.
Western cultures typically associate being "high" or "up" with
affluence and
success (e.g., things are looking up, profits are rising), and "low" and "down" with
poverty and
unhappiness (e.g., I'm feeling down today, I can't sink any lower). Now, imagine that you're as far down as you can get. You're lying on the ground. But if things get even worse, you will go
under the ground. Now you're in the
dirt. Now, in addition to everything being as bad as it can be, I run off with your sheep and you start guzzling
woolite for solace. Now you seriously can't get any
lower; you're beneath the dirt and on the
bedrock. Even if you took a
shovel and
tried to dig yourself deeper in, you couldn't get past that. You have hit the rock bottom.