"And swearing 'til my very _roof_ was dry,"
transmogrified into--
"And swearing 'til my very _tongue_ was dry."
Now, why "this lame and impotent conclusion?" What can be a more common
expression than the "roof of the mouth?" and it is just the part which is
most affected by a sensation of dryness and pricking, after any excitement
in speaking, whereas the _tongue_ is not the member that suffers!