Here's why it's used:
* Specificity vs. Generality: It's more specific than saying "sometime next week" but still avoids committing to a definite date.
* Flexibility: It allows for planning without locking in a rigid schedule.
* Humor: It can be used playfully, especially in situations where the exact date is unimportant.
Similar phrases:
* "Sometime next week"
* "Next Tuesday"
* "In a week or two"
While the exact origin remains unknown, the use of this phrase likely developed organically as a way to communicate a future date with a degree of flexibility.