Q From Erol Bozok, Turkey; a related question came from John Russo: I am wondering if you could explain what wax poetic means and its origin. There is a US-based music band called Wax Poetic and I have heard the phrase or idiom on a couple of other occasions but have never been able to figure out what the speaker meant.
A These days, the verb to wax — if we leave aside such associations as polishing cars and removing hair from legs — mostly turns up in connection with the phases of the moon. The moon waxes when its illuminated face grows larger from new moon to full moon, then wanes until the cycle starts again.
At one time wax was the usual verb meaning to become bigger, but from the fifteenth century onwards it was progressively replaced by grow, so that it survives now only in discussions of the moon or in set phrases such as your wax poetic and similar formations — wax eloquent, wax lyrical, even wax sententious.
I agree this is an idiomatic form that’s not easy to understand. In everyday usage, in which such set forms are bordering on cliché, it means merely to communicate in the way described. So wax poetic means only “speak or write poetically”. Sometimes there’s a hint that the person is doing so increasingly — becoming expansive in his language, figuratively increasing or enlarging the specified quality — but that’s present rarely enough that a connection with the “grow” sense of wax can’t be assumed. However, a link exists, since the usage perpetuates one old sense of the verb, “become or turn”, with a nod to another, in which wax before an adjective meant to gradually increase that quality or become it.
It may be significant that the first edition of the Oxford English Dictionary, whose entry for wax was published in 1926, didn’t include this idiomatic sense of the verb and it had to wait until 2006 before it was added to the online entry. The first example in this sense is dated 1842, but there’s no instance included of wax poetic. The earliest I can find is in Sir Henry Morton Stanley’s book of 1872, How I Found Livingstone: “One could almost wax poetic, but we will keep such ambitious ideas for a future day.”
It certainly seems that this construction became common only after the literal usage of the verb had declined to almost nothing.