Neologisms

A funny email from the Scientist on new meanings of old words provoked thought on the vagaries, nay the sheer bizarreness, of USAmerican English. There is the oft-cited gem: 'momentarily' being used to mean 'in a moment' as oppsoed to what it really means, 'for a moment'.

In all the reading I've had to do of much scholarly writing subscribing to USAmerican standards, I've encountered some words that make me stop and go WHYYYYYY? before I can move on. [Perhaps I merit the teasing BSW inflicts on me for trying to be more English than the English. Last week he was delighted because I said who all are, instead of who is, and proceeded to spend twenty minutes talking about Indian English.]

The one that stands out the most is "undergird". What, by Darwin's big toe, is that supposed to mean? It seems to be a fancy way to say underline. Only, there already IS a fancy way to say underline, especially metaphorically, i.e. underscore. Doing some research I discovered, according to dictionary.com, what 'gird' means. [I apologise, I didn't being my OED. Besides, I think dictionary.com provides a fairly accurate estimation of the language as most widely used and interpreted in this country.]

1.	to encircle or bind with a belt or band.

2.	to surround; enclose; hem in.

3.	to prepare (oneself) for action: He girded himself for the trial ahead.

4.	to provide, equip, or invest, as with power or strength.

Now, in the light of this, I'm trying very hard to find how one can undergird anything in the first place, and secondly, which of the possible interpretations would fit in the following sentence:

"We'd spoken of the gradual disintegration, during the late fifties and sixties, of the prosperity which undergirded that stability"

Lets see:

1. under-enclosed?
2. under-encircled? under-hemmed-in?
3. under-prepared oneself? [Ok, this one isn't relevant.]
4. under-equipped with strength?

I think the word wanted here is underlay, or even formed the basis of.

Votes?