The cavity beneath the junction of the arm and shoulder.
She stood a couple of feet away from me […] exposing an edge of brassière strap at her armpit―exposing the armpit itself, a muscular hollow shadowed with fine stubble and faintly streaked with talcum.
Somewhere or something unpleasant or undesirable.
the armpit of America
From Middle English armpitt, arme-pitt, armput, equivalent to arm + pit. (something unpleasant): Presumably a reference to underarm body odor.
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