In the olden days it's a cheaply made sack that was used to carry things. Think bags of potatoes at the old General Store that you might see in a classic western.
http://www.dreamstime.com/stock-phot...-image16956900
That said, if you wore a burlap sack Speedo, hopefully it would have a nice smooth lining, otherwise you'd be quite uncomfortable.
Basically saying you're so hot you could wear a sack and you'd look amazing.