Worked through “U”? (4)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

Triple definition, though the first two are related meanings. As in working (doing) a stitching pattern in sewing or knitting; as in “I’m through with it” = done = finished; or as in “the done thing” = acceptable behaviour, for example the perceived class distinction between “U” and “non-U” usage.