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. 

It began as a euphemistic expression of “Jesus,” used to avoid taking the Lord’s name in vain, which was considered inappropriate. Euphemisms like this were common in American English, allowing speakers to show emotion while keeping social decorum.

“Gee whiz” appeared in print in the early 1900s and became especially popular in the 1920s and 1930s. Comic strips, radio shows, and films often gave the phrase to youthful or wide-eyed characters, cementing it as a symbol of innocent astonishment.