armpit (n.)

mid-14c., "hollow place under the shoulder," from arm (n.1) + pit (n.1). Arm-hole (early 14c.) was used in this sense but was obsolete by 18c. Another Middle English word was asselle (early 15c.), from Old French asselle, from Latin axilla. Colloquial armpit of the nation for any locale regarded as ugly and disgusting was in use by 1965.