It is a traditional 8-yard tartan kilt in a heavy wool fabric. Most guys can't readily tell the diference between 13 oz and 16 oz when they are wearing it, particularly after a few pints! At one point I believe it was 16 oz and up, then it was agreed that 13 oz was the strict minimum for a tank, but it has to be a full 8 yards (not 5 to 7) and worsted wool (not PV).

It is a term only used by Xmark'ers.