An idiom.

It means "completely pure" or "untainted", usually in the moralistic sense. The snow is "driven" per the Old English meaning of "drifted" or "driven by wind", not to be confused with snow that has been "driven on" by cars.

The phrase has a Shakespearean quality, though it doesn't appear verbatim in any of his plays. In The Winter's Tale though, a street vendor sings a song that begins "Lawn as white as driven snow / Cyprus black as e'er was crow". ("Lawn", in this case, being a type of linen, and "Cyprus" a crape-like fabric. Still with me? Okay.) In Macbeth, Malcolm remarks "black Macbeth will seem as pure as snow".

In a nice twist on the cliche, 50s radio host Tallulah Bankhead once described herself as "pure as the driven slush". (Bankhead, by the way, was also one of Hollywood's first "out" actresses.)

Another great Tallulah line? "My daddy warned me about men and booze but he didn't say a word about women and cocaine!"

The odd thing about the line "Pure as the driven snow" is that its meaning isn't understood the same way it was ages ago.  Originally, "driven" could mean "windblown," but that's not how most modern people would see it. 

When I first heard this idiom, I thought, "Driven snow isn't pure."  For myself as well as most others (when they think about it), "driven snow" conjures up images of packed ice on the side of the road tainted with mud and slush.  We still use it - we just don't think about how meanings have changed.

It's up to you to decide whether or not this is a metaphor for changing times.  Personally, I don't want to dig too deeply into something coincidental.

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