I'm plum/plumb out of sugar

[Echo]

My mom used to say "I'm plum out of sugar" which I find
funny, considering plums are a sugary food.

Then my wife used it today!

Help me win my argument! She trusts you!

Isn't it "plumb" and not "plum"?

[Peter Duncanson [BrE]]

Yes. "Plumb" and "plum" are pronounced the same so it's not surprising
that some people spell it one way and others the other.

The OED says:

plumb, adv. and adj.

....
2.
a. As an intensifier: completely, absolutely, quite. In modern use
often in negative context. Now chiefly N. Amer. colloq.

The quotations given show various spellings:

1588 T. Hughes Misfortunes Arthur ii. iv. 21 The mounting minde
that climes the hauty cliftes..Intoxicats the braine with guiddy
drifts, Then rowles, and reeles, and falles at length plum ripe.
1738 J. J. Berlu Treasury Drugs Unlock'd (ed. 2) 67 The best
[jujubes] are plumb-full of Pulp, and come from Italy.
1787 F. Grose Provinc. Gloss., Plum pleasant, very pleasant.
1846 H. Kerchival in S. F. Smith Theatr. Apprenticeship App. 213
Long before the time arrived..the house was plum, chock full—full
to overflowing.
1857 Olney (Illinois) Times 25 Dec. 1/5 He wur plum crazy fur he
jist spit in his hands an leaped over the frunt uv the pulpit.
a1861 T. Winthrop John Brent (1864) xxviii. 296 When we got
here, I paid their tickets plum through to York out of my own
belt.
1897 R. Kipling Capt. Courageous 21 You've turned up, plain,
plumb providential for all concerned.
1914 E. P. Stewart Lett. Woman Homesteader v, I..told Mr.
Stewart about my talk with Patterson, and he said, ‘Wooman, some
day ye'll gang ploom daft.’
1926 ‘R. Crompton’ William—the Conqueror v. 89 Poor woman! She's
sure plumb crazy!

More recent quotations use "plumb".

http://oxforddictionaries.com/definition/english/plumb?q=plumb

plumb
....
adverb

1 informal exactly:
"trading opportunities plumb in the centre of central Europe"

[as submodifier] North American extremely or completely:
"they must both be plumb crazy"

--
Peter Duncanson, UK
(in alt.english.usage)

[Ian Jackson]

In message <7g00o8p8341hm0p9j...@4ax.com>, "Peter
Duncanson [BrE]" <ma...@peterduncanson.net> writes

I'm guessing that "plumb" - in the sense of "exact", "surely",
"certainly", "truly", "completely", and similar words) - comes from the
use of a plumb-line to ensure that things are
exactly/certainly/truly/completely vertical.
--
Ian

[Peter Duncanson [BrE]]

That seems to be the case. "plumb, adv. and adj." comes from the noun:

plumb, n.1

Etymology: < Anglo-Norman plom, plombe, plomme, plum, plumb, etc.
and Middle French plom, ... Latin _plumbum_ lead, ball
of lead used as a missile....

1.
a. A ball or piece of lead or other dense substance, attached to a
plumb line or quadrant for determining the vertical. Cf. plumb
bob n. Now rare.

a1400 (?a1325) Cursor Mundi (Vesp.) 2247 (MED), Wit cord and
plum [a1400 Fairf. plumme; a1400 Göt plumbe], þai wroght sa hei.
....

[Bill McCray]

There is also "Plumb the depths". Grammar-monster.com says:

"‘To plumb the depths’ means to let someone or yourself down severely.
It also means to explore intensely. This saying has a nautical origin.
It derives from when a sailor would be required to lower a lead weight
into the sea on a length of rope to determine the depth of the water.
In doing so, he would be required to lower the weight to the very bottom
of the sea. You are perceived as being as low as the lead weight when
you let someone(or yourself) down badly. Also, extending the rope all
the way to the bottom means that you have kept going until you have your
answer, and this is why ‘to plumb the depths’ also means to explore
intensely."

Bill from Kentucky