mid-14c., from arm (n.1) + pit (n.1). Arm-hole (early 14c.) was used in this sense but was obsolete by 18c. Colloquial armpit of the nation for any locale regarded as ugly and disgusting was in use by 1965.
1. The nurse shook the thermometer and put it under my armpit.
2. She was carrying her handbag under her armpit when I saw her.
3. Detroit was considered the armpit of America in the 1970 s.
4. Please put this thermometer under your armpit and let me feel your pulse.