What picture do you get when you see or hear this word? Perhaps a cool customer, one with chin up and chest out but as calm as Jeeves, someone who, when pitted against a problem, does not take things lying down but does not throw his (or her) weight around, someone who surveys the situation, senses the gravity of the circumstance but always keeps head on shoulders, takes the lead, and gives a measured response? And perhaps someone with a plummy accent?
The look and sound of the word seem to go well with the sense. The pl is seen in calm, measured words such as please, pleasant, plan, and plush, as well as weighty words such as plot, plop, plug, and plough, along with an assortment of other kinds of words such as pluck, plural, and plight. The calm /m/ of the end with its thoughtful nasal hum can also be seen to be leaving something unsaid – that b there. Ah, that b. Look: first you see it hanging upside down p, then it is right side up but in two pieces lo, and with the calm hand of intervention m it ends up as it should be, but tamed. Yes, this word gets from a to b so calmly you don’t even notice you’re at b.
And where did that b come from? Originally from Latin plumbum, which is the reason the chemical symbol for lead is Pb. If, like a surveyor, you take a properly shaped piece of plumbum and dangle it from a string as a weight, you will find it will find gravity for you quite nicely and tell you exactly which way is up. We may call such a device a plumb bob (sounds like a Hallowe’en game) or a plummet. If you happen to be French, however, “lead” is plomb and if you follow the lead of the lead – not throwing it around, but just being as upright as it is – you are à plomb. So this French prepositional phrase meaning “in vertical position” is now a noun in English meaning, roughly, “as cool as a cucumber”.
This noun, incidentally, is found most of the time with the word with usually two or three words before it: with grace and aplomb, with equal aplomb, with as much aplomb, with considerable aplomb, with characteristic aplomb, with the same bleary-eyed aplomb, et cetera. This is not so surprising, since it expresses a manner of doing things (and doing things with manners). It could have been an adjective (like adroit) or an adverb, but somehow it ended up a noun. Perhaps because it expresses such stability. And perhaps because a plum is, after all, a noun.
Thanks to Jens Wiechers for asking for aplomb.