i Register
In some senses, armpit is marked as derogatory, colloquial. Watch for register when choosing this word.
noun
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.
Unilever brought its Rexona deodorant to China a decade ago, dreaming of a market with 2.6 billion armpits.
Somewhere or something unpleasant or undesirable.
the armpit of America