Sa maison est propre comme un sou neuf.
sah may-ZAW ay PROH-pruh kum uh SOO NUHF.
Her house is neat as a pin.
This post is dedicated to my mother, who requested the phrase and who has made a lifelong practice of it!
The original meaning of neat, in English, was clean, from the Latin nitidus (bright, clean) via the French net (clean, pure, untarnished). So neat as a pin originally suggested shiny, more than tidy, as we tend to use the English word today.
And that’s what the French, propre comme un sou neuf, says: clean as a new coin. Propre in French also suggests clean, but more in the sense of proper, correct, as things should be–tidy, in fact.
As for un sou, that’s the name for an ancient coin in France: one-twentieth of a pound, une livre. That’s like a shilling in the former British monetary system.