pure as the driven snow
Morally unsullied, chaste, as in She's just sixteen and pure as the driven snow. This simile dates from the late 1500s, although driven, which means “carried by the wind into drifts,” was occasionally omitted. It is heard less often today.
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.