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.
by Buckeye Starrider May 03, 2007
deadset my heart and soul, clean as a kiwi cranny-bum
choice bro,now that's as clean as your mother's reputation
by S & A May 18, 2007