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.
Fogey/fogy /fougi/ sl. (early 18C+, orig. Scot) old-fashioned, stuck-in-the mud.
Person with old fashioned ideas which he is unwilling to change: Come to the disco and stop being such an old fogey!
You think me an old fogeyand an old tory, his thoughtful voice said. I saw three generations since O’Connel’s time. I remember the famine. Do you know that the orange lodges agitated for repeal of the union twenty years before O’Connel did or before the prelates of your communion denounced him as a demagogue? You fenians forget some things. (James Joyce, Ulysses. Penguin Books,1992. p. 38)