I've been reading Gideon the Cutpurse recently. It's a decently-well-written story about two kids in modern England who accidentally travel back to the mid-18th-Century. (Think The Golden Compass with somewhat less imaginative world building.)

The word in question is "bottom." As used by those native to the 18th Century in England, it means something like "courage" or "gumption." From what I can tell, the now-common meaning "derriere" did not come about until nearly the end of the 18th Century. The problem I'm having is that I cannot figure out how the "courage" meaning came about for a word that has its roots strongly in "base," "fundamental," or "ground." (It also seems a bit odd that whoever coined "bottom" as "derriere" clearly thought of the rear-end, and not the feet as the "bottom" of the human. I guess he sat a lot.)

Comes from ME botme and OE botm, but so far as I can see it has never meant 'courage.' Perhaps it's a corruption (by the author?) of 'bottle' when used in that way?

I think the author may be having a laugh... use the Search Inside function of the Amazon link given above...
I expect every member of the party to show their bottom on this journey.
"Well, I admire your bottom, young sir," said the innkeeper's wife, ignoring Kate's snigger.
In which the parson preaches a curious sermon and the party shows their bottom
Fancy liking the French... but he had bottom.

The B&M OED has a definition with citations from the late 18th to the mid 19th centuries: Physical resources, 'staying power', power of endurance ; said esp. of pugilists, wrestlers, race-horses, etc.

@Pook: the author's doing exactly that. It's meant as a joke to us from the 21st Century. I was just hoping that it really was used in the way the 17th Century people used it. It feels cheap as a single entendre.

Assuming you came up with the 'courage' meaning just from context and you read it with the 'staying power' meaning that the OED offers it makes sense in a 17th century context and gets your 20th century second meaning if you think the author really intended that.

It seems to me that 'staying power' quite aptly describes my derriere !!

Think of other descriptions for someone with staying power or power of endurance: solid, of fixed purpose or immovable or unshakable.
"Having bottom" referring to a solid foundation or a heavy enough base to prevent tipping over makes sense in this context.

Well, dare I say it, I'm glad we got to the bottom of that then...