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 I Saw U2 Live Twice May 4, 2007
choice bro,now that's as clean as your mother's reputation
by S & A May 18, 2007