bye-bye

The journey of "bye-bye" begins with the late 16th-century contraction "goodbye," an elision of the pious farewell "God be with ye." Over time, the religious connotation faded, leaving a standard term for parting. The further shortening to "bye" in the 18th century created a more casual form. The playful, child-friendly reduplication "bye-bye" emerged in the early 19th century, leveraging repetition—a common linguistic device for creating affectionate, simple, or diminutive forms, especially in nursery language and informal speech.

This is a reduplication of the word "bye," a shortened form of "goodbye." "Goodbye" itself originates from the phrase "God be with ye," a contraction and alteration of the Middle English phrase "God be with you." The reduplication is a common feature in child-directed speech and informal language.