YOY stand for “Yarn Over” Yarn, it is an innovative yarn spun by Fibre Arts Shed in New South Wales, Australia. This is the description from their website:

50% reclaimed mill fibre + 50% “new wool”. We call this range our “Yarn Over” Yarn because we are using some of the fibre over again! Did you know that it is normal to experience a loss of 40% of weight for most yarns from fleece to final product? Some of that weight is lanolin and dirt but a lot of it is just fibre that falls/flies off the machinery during the yarn making process. Normally, this fibre is bagged up and disposed of. But we have reclaimed some of this fibre and blended it with a “new wool” to produce this lofty Aran/10-ply weight woolen spun yarn.

Due to the nature how this yarn is made each batch is unique. The 50% new wool in this batch is Cheviot and the base colour is a darkish soft brown.