Origin of "Plumb" to mean "absolutely"

"plumb" as far as I know is a predominantly American usage, as in "That was just plumb crazy!"

I thought plumb meant some kind of weight in bricklaying or such like, so how did it come to mean "absolutely"? What was its origin and history?

Edit: After Callithumpian's answer, I have a little addition:

Why is it a US usage chiefly, and why do not the British use it as much?

Solution 1:

Etymonline indicates that the "completely" sense of the word was an extension of the "exact measurement" sense of the word and dates this shift back to the mid-18th:

plumb (n.) c.1300, "lead hung on a string to show the vertical line," from O.Fr. *plombe, plomme "sounding lead," from L.L. *plumba, originally pl. of L. plumbum "lead," the metal, of unknown origin, related to Gk. molybdos "lead" (dial. bolimos), probably from an extinct Mediterranean language, perhaps Iberian. The verb is first recorded late 14c., with sense "to immerse;" meaning "take soundings with a plumb" is first recorded 1560s; figurative sense of "to get to the bottom of" is from 1590s. Plumb-bob is from 1835. Adj. sense of "perpendicular, vertical" is from mid-15c.; the notion of "exact measurement" led to extended sense of "completely, downright" (1748), sometimes spelled plump or plunk.

Edit re: British vs. American use:

I found this newsletter on British plumb bobs and noticed that another word used for the tool in several publications was plummet rather than plumb-bob. Perhaps plummet was a common enough name for the tool in England that it prevented the development of the extended sense of plumb as occurred in the U.S. This is only a guess.

Edit re: confusion with plum:

After seeing the OED references in @Simon's answer, I agree with @Peter's comment that there seems to be some confusion between the etymology of plumb and plum as intensifiers. I found this on plum at Etymonline:

plum O.E. plume, early Gmc. borrowing (cf. M.Du. prume, O.H.G. phruma, Ger. Pflaume) from V.L. *pruna, from L. prunum "plum," from Gk. prounon, later form of proumnon, from an Asiatic language. Change of pr- to pl- is unique to Gmc. Meaning "something desirable" is first recorded 1780, probably in ref. to the sugar-rich bits of a plum pudding, etc.

Some of the OED references seem to be examples of plum being used to mean "something desirable" rather than misspellings of plumb meaning "completely."

Solution 2:

Lots of background and examples in other answers, but I really don't see anything at all unusual in this type of 'specialised meaning'.

Plumb weights are made of lead (Pb=plumbum), and they give straight vertical lines. That explanation is straight up (absolutely true), and it's straight to the point (absolutely focussed).

Does anyone ever wonder why we use straight in such figurative senses? I can't see why the American use of plumb is much different.

Brits don't say it simply because it calls to mind American 'hicks from the sticks' who are invariably plumb loco.

Solution 3:

Callithumpian's answer is all you would need; except for an actual answer to 'how did it come to mean...?' Surely 'plumb vertical' is a builder's term, meaning 'precisely vertical as measured by plumbline', and it was taken up by outsiders who assumed it just meant very (I have seen it as plum as well as the variations in the dictionary).

Solution 4:

I find the 'plumb' arguments abstruse, myself.

If things can be 'peachy keen' why can't they be 'plum crazy'?