The Collaborative International Dictionary of English v.0.48:
Armpit \Arm"pit`\, n. [Arm + pit.] The hollow beneath the junction of the arm and shoulder; the axilla. [1913 Webster]WordNet (r) 3.0 (2006):
armpit n 1: the hollow under the arm where it is joined to the shoulder; "they were up to their armpits in water" [syn: armpit, axilla, axillary cavity, axillary fossa]