Basically, this term means that you can't get something any cleaner (or purer) than it is. It's also meant as a compliment by saying that somebody's mother is not a skanky whore or streetwalker. This term appeared in the young adult edition of the novel adaptation of the 1979 sci-fi thriller"Alien".
George the janitor tidied up the restroom so well it's not simply spic and span, it's as clean as your mother's reputation.
This is a saying used by people in the southern U.S. to imply that a person or organization is without corruption or completely honest.
Buford: I hear that the 34th Precinct of the Atlanta Police Force has Cops on the take.
Bubba: No that's not true,I hear that they are Clean as a Hounds Tooth.
When you take a big shit and you go to wipe but when you look at your toilet paper after the wipe there is so shit streaks on it. So your butthole is as clean as a whiteslip.