I think the difference is significant. I have seen some of the foul weather gear have something like this built in. But I don't think I would wear that in site of anyone! And I am not vain. I find if I expect to be in foul weather I wear my hiking/rock climbing pants as they are nylon and dry very fast.you know silly me I googled it, and you'd be surprised... most were references to PJs Norman Rockwell era... the others well, not a pretty picture. I'll just say it was in the urban dictionary. Thanks for the actual link. By the way you referenced butt flap not bum flap, a distinction of splitting hairs but a difference just the same, that could be the source of my failure in google search though.
So this would be a "seat" you bring with you, I suppose?