WS stands for wrong side, the face of a knitted or woven fabric intended to be hidden on the inside of a garment. Pattern instructions use it to specify which side should face the maker during a particular row or operation.
Heard on the job
Purl all stitches on the WS rows to maintain stockinette.
Insider tip
Always mark the WS with a locking stitch marker if the fabric is reversible or hard to distinguish.