American slang for one's last shilling. A miner's term, called in England the “stone-head,” and in America, the “Bed-rock,” the hard basis rock. When miners get to this bed the mine is exhausted. “I'm come down to the bed-rock,” i.e. my last dollar.
“ `No, no!' continued Tennessee's partner, hastily, `Ill play this yer hand alone. I've come down to the bed-rock; it's just this: Tennessee, thar, has played it pretty rough and expensive, like, on a stranger ... Now what's the fair thing? Some would say more, and some would say less. Here's seventeen hundred dollars in coarse gold and a watch- it's about all my pile- and call it square.' ” —Bret Harte; Tennessee's Partner.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894