what is a posie flower
The ring around the rosies is the red ringed rash that was the sure sign of having the plague. If you developed a rash it wasn't serious, unless a red ring appeared around the spot. A pocket full of posies is the flowers brought to a funeral. Or the fact that the smell of decaying bodies was so bad that people began carrying flowers in their pockets to try to ward off the smell. It's ashes ashes (not atishoo) which is because they tried to burn the bodies, rather than bury them.