Etymology
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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.

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Definitions of armpit

armpit (n.)
the hollow under the arm where it is joined to the shoulder;
they were up to their armpits in water
Synonyms: axilla / axillary cavity / axillary fossa
From wordnet.princeton.edu