The phrase "gee whiz" is an exclamation of surprise, amazement, or enthusiasm. Its origin can be traced back to the early 20th century in the United States. 

"Gee whiz" is a euphemistic expression derived from "Jesus," used as a way to avoid taking the Lord's name in vain, which was considered inappropriate or disrespectful. Such euphemisms were common in American English to maintain decorum while still expressing strong emotions. 

The phrase started appearing in print around the early 1900s. It became particularly popular in the 1920s and 1930s, often used in comic strips, radio shows, and movies to convey a sense of innocent astonishment or excitement, especially among younger characters.