As pure as the driven snow
'Driven snow' is snow that has blown into drifts and is untrodded and clean. Examples of the precise text 'as pure as [the] driven snow' aren't found in print until around the start of the 19th century. Nevertheless, we have to thank Shakespeare for this popular simile. The complete phrase 'as pure as the driven snow' doesn't appear in Shakespeare's writing, but it almost does, and he used snow as a symbol for purity and whiteness in several plays. In The Winter's Tale, 1611:
Autolycus: Lawn as white as driven snow.
In Macbeth, 1605:
Malcolm: Black Macbeth will seem as pure as snow.
Of course, the tradition of brides wearing white in many cultures stems from the association between the colour and purity. This was referred to as early as the 1400s, as in John Lydgate's poetry for example, circa 1435:
Alle cladde in white, in tokne off clennesse, Lyche pure virgynes.
An alternative derivation of this simile has been proposed that originates from an altogether different source. Mediaeval tanners used animal faeces in the leather tanning process - specifically dogs' droppings, to which they gave the incongruous name 'pure'. Some have speculated that 'pure' referred to the white form of the said stools that used to be more commonly seen and that 'as pure as the driven snow' comes from that association. It doesn't; the 'pure' name came from the purification of the raw leather caused by the enzymes present in the excrement and has nothing to do with 'as pure as driven snow'.
See other 'as x as y similes'.
See other phrases and sayings from Shakespeare.