The normally meaning for "ordering a pair of overalls" is just that but in the cowboy sense it does indeed take on a different meaning altogether. Overalls is cowboy slang for leg restraints normally used to hold a cow still while it is being branded. This has of caused developed over the years into other meanings. The latest being used by gay cowboys into S&M. So by ordering a pair, it looks as if this particular one is in for a rough ole time. Yee Har.