wax poetic

become poetic, begin to recite poems

  

Whenever you read the Psalms, Andy begins to wax poetic.

Wax is also an intransitive verb. In one sense it means to increase, as in the moon waxes full this evening, and in another sense to pass into a mood or state. In the latter sense we can understand the phrase wax poetic (or eloquent, lyrical, philosophical or sarcastic) and this raises doubt about its status as an idiom, i.e., it can be taken literally. The Canadian Oxford Dictionary suggests that wax 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.