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So wound up by a situation you have amassed the pent up energy like that of a coiled spring. Fit to bust is the pinnacle of that stored energy. Once triggered the energy released results in a cascade of emotions.
The build up to Christmas day, starting with decorations, wrapping presents, the tree, pies for santa and sleeping on Christmas eve knowing that in the morning you can open presents.

By morning children are 'fit to bust' The reaction of the children is the release.
by aoides November 27, 2013