We have to bring out the big guns for the origin of this one. Chaucer, Shakespeare and the Bible all contain versions of white as snow.
From Shakespeare’s Hamlet, 1602:
… What if this cursed hand
Were thicker than itself with brother’s blood,
Is there not rain enough in the sweet heavens
To wash it white as snow? …
The King James Version of the Bible, 1611, has this in Daniel 7:9:
I beheld till the thrones were cast down, and the Ancient of days did sit, whose garment was white as snow, and the hair of his head like the pure wool: his throne was like the fiery flame, and his wheels as burning fire.
The simile ‘as white as …’ is found in many forms in early English. The white thing in question was variously milk, flour, whale bone, blossom or snow.
To find the earliest use of ‘as white as snow’ we need to go back to Old English texts, like this from the West Saxon Gospels, circa 1150:
Hys reaf wæron swa hwite swa snaw
A reaf was a type of loose toga-like garment, so a rough translation of the above would be ‘The cloak he was wearing was as white as snow’.
We might imagine that ‘as white as snow’ was the precursor to ‘snow-white’. The fairy tale Snow White was collected by the Brothers Grimm in the 19th century, but the term snow-white is much earlier and pre-dates ‘as white as snow’ by several hundred years. It is recorded in Old English from circa 1000 and was used in the 14th century by Geoffrey Chaucer in The Canterbury Tales – The Second Nun’s Tale:
Valerian said, “Two crownes here have we,
Snow-white and rose-red, that shine clear,
Both ‘snow-white’ and ‘as white as snow’ were in common use by Shakespeare’s day. So much so that a single word was coined to convey the same meaning. This was recorded by Henry Cockeram in
The English dictionarie, or an interpreter of hard English word, 1623, where he defines the word ‘nixious’ as meaning ‘as white as snow’.
See other ‘as x as y similes‘.