The exact origin of the phrase is unknown, but it likely evolved from the word "joke" itself. The word "joke" has been around in English since the 16th century, and the shortened form "joshin'" is likely a natural progression in spoken language.
Here's a possible timeline of the phrase's evolution:
1. "Joke" (16th century) - The word itself is established.
2. "Joking" (17th century) - The verb form becomes common.
3. "Just joking" (18th century) - The phrase used to clarify a playful intent emerges.
4. "Just joshin'" (19th or 20th century) - The shortened, informal form becomes popular.
It's important to note that the phrase's popularity likely spread through oral communication, making it difficult to pinpoint an exact origin. However, its widespread use today suggests it has been part of the English language for a significant time.