Lots of senesible answers to my "silly" question--thanks! Looks like "working clothes" has most support; the reason I wasn't entirely convinced is that "pants" are mentioned specifically in the first verse.

My great-grandfather used to walk five miles or so from Blakenall (S. Staffs) to the Wyrley [rhymes with "early"] Grove pit every day in the early years of the century. In those days you'd come back dirty and wash at home; even if you stripped before working, your clothes would get dirty on the inside when you put them back on. Granddad would tell me how his father would often be sent back half-way through the week with just a couple of shillings (10p) to feed the family (abut half-a-dozen kids, I think; though I keep discovering more great aunts & uncles). He never mentioned blacklegs: maybe there weren't any in those parts at the time?

Thanks again, folks!
Steve