The phrase "not only" is used to introduce the first of two related ideas, emphasizing that more than one thing is being discussed. It often pairs with "but also," highlighting a relationship between the two parts of a sentence, typically to suggest that the second idea adds to or contrasts with the first.
The phrase "not only" has roots in Middle English, where "not" serves as a negation and "only," derived from the Old English "ānlic," indicates exclusivity. Together, they convey a notion of negating exclusivity while introducing additional information.