One can wax poetic, lyrical, eloquent, philosophical, etc. It's all pretty much the same thing. But finding the origins of this phrase is a bit tricky. One sense of wax (as an intransitive verb) is to pass into a mood or state. In this sense one could understand the phrase wax poetic (or sarcastic, or whatever) and this would certainly lead one to doubt whether this phrase is an idiom (as I had been thinking) because it can be taken literally. The Canadian Oxford Dictionary suggests that wax (in this sense) has a Germanic origin: wachsen (to grow). The interesting thing about this relationship is that wachsen never has an attribute, i.e. neither wax poetic nor grow old translate. The Encarta online dictionary's suggestion is that this sense of wax is from Old English weaxan, and ultimately from an Indo-European base meaning “to increase,” which is also the ancestor of English "augment" and "eke." The American Heritage Dictionary agrees, adding that this word is an offshoot of the Indo-European Language family's root fragment "aug", from whence we get words like augment, inaugurate, auxilliary, august, and auction. Hope that helps a bit!