I heard this all the time growing up, have no idea what the ''thumb'' part stands for.
Maybe it's the thumb that is a different take so when the thumb rules and not the fingers it's rendered useful if it's the thumb, like it's the ......."new rule of this territory, '', or the ''rule of the home'', depending upon who's in charge of that rule at that time, which ever WORKED BEST, like the best answer ! ! ! ! !
So I'd say it's the best answer from all those rules made at some moment in time, whatever worked best.
at 8:58 AM on Nov. 9, 2010