From Longman Dictionary of Contemporary Englishthe armpit of somethingthe armpit of somethingAmerican English informal the ugliest or worst place in a particular city or area Dale says Butte is the armpit of Montana. → armpit
Examples from the Corpus
the armpit of something• It is the smell of Pears shampoo and Mum deodorant leaking milky white through the armpits of my blouse.• Dale says the city of Butte is the armpit of Montana. 