No, we haven't got a clue either.
This is one of the 7a Virtues whose statues adorn the Library at Unseen University; but this is Ankh-Morpork, remember. It doesn't matter that seven-and-one Virtues outnumber the Seven Deadly Sins by one. In Ankh-Morpork, which gang do you think is going to win?
Tubso, like her sister Bissonomy, is a Virtue honoured by so few people (and even that was so long ago that nobody knows what it is any more, nor how to be properly Tubsonian or Bissonomic) that there is now no-one in Ankh-Morpork, not even from the Guild of Historians who can explain what the virtue was. You may be sure none of the Seven Deadly Sins has fallen out of memory in the same way. Seeing as this is Ankh-Morpork, they´ve probably invented a couple more. Just for fun, you understand.