armpit (n.)youdaoicibaDictYouDict[armpit 词源字典]
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. [armpit etymology, armpit origin, 英语词源]