To wax poetic means to use flowery or expressive language to describe a relatively mundane topic, or using heightened, formal language to communicate a simple thought. A cook might wax poetic about his latest dessert; "overflowing with the rich chocolate lava poured from the volcano of love itself." A car owner might wax poetic about the angelic hum of a perfectly tuned engine. Some people could also be said to wax philosophical about their newest hobbies, or wax lyrical about their latest successes. Of all these expressions, however, waxing poetical appears to have survived the longest in modern usage.
The word wax in the expression does not refer to any waxy substance, like candle wax or car wax. Instead, it is an archaic word meaning "to grow." Some people still refer to the "waxing and waning" of the moon, for example. A waxing moon is growing in apparent size from a sliver to a half-moon and finally to a full moon. A waning moon goes through the opposite phase, reducing in apparent size from full to "new," or completely dark. It was also common in ancient times to refer to a person increasing or waxing in character or in physical size. Early English literature does contain references to the word weaxen, an Old English form of "waxing".
The use of expressions such as waxing lyrical or waxing strong can be traced back many centuries, but the first use of wax poetic is thought to be in a book written by the famous English doctor Stanley Livingstone in 1872. Livingstone was referring to the temptation to wax poetic about his experiences as a world traveler and physician. By waxing poetical about his personal experiences, he could be seen as romanticizing the past or using unnecessarily florid language for the sake of art alone.
Essentially, whenever a person waxes poetic on a topic he or she is passionate about, there is always a temptation to exaggerate or glorify the subject at hand. Sometimes this heightened and expressive language is effective. Indeed, many successful advertising campaigns are based on the concept of waxing poetic about a new food product or exotic vacation destination or seductive new fragrance. When done effectively, waxing poetic about an experience or product can help the audience envision it in their own minds. To wax poetic would mean to grow or expand one's language from the ordinary to the extraordinary, from the expected to the unexpected.