armpit

noun2

1. •
   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. — 2009
2. •
   (informal, disapproving) Somewhere or something unpleasant or undesirable.