There are quite a number of similar minced oaths here. A number are very surprising to me.
By the way, a minced oath is a way to mince your words, using a shorthand for another phrase which might be offensive, especially on religious grounds.
So what what the purpose of the expression? The person speaking it stated that he/she did not have any idea why he/she said that. I would suggest it was one of these:
- Expressing pain at the sitting motion
- Expressing a state of being tired or weary
- General expression of frustration with life's challenges.
- State of a release of emotion and stress - upon relaxing
- Sitting down really hurts!
- I am very tired!
- Oh, life is rough!
- Oh, what a relief it is!
Share other suggestions in the comments.