When I was taking typing in high school, the light, translucent paper
that was used for the carbon copy (behind the carbon paper) was called
'onion skin'.  To me, the term refers to that kind of typing paper.

There is a seller on eBay that has rolls of paper she's referring to
as onion skin.  The seller's name is cadpaper.  The listing is entitled
"Sketch paper, onion skin 50 yd rolls all sizes".

Lorna Joy Shashinda