Ok,, I had planned to add one thing to my previous post. But forgot.

I think the Bibles reference to 'sack cloth' is just symbolic.

I think people wore 'sack cloth' back then when they were too poor to wear anything better, or too sad, or serious, to care to wear anything better. What we might understand better if they said 'wearing rags'.

In the context of modern time, I think it could describe someone who just dresses for utility. no fancyness. because they dont feel it is a time for 'fashion'. or, someone who cant afford better than basic clothing.