From WordNet (r) 2.0 (August 2003) [wn]:
armpit
n : the hollow under the arm where it is joined to the shoulder;
"they were up to their armpits in water" [syn: axilla,
axillary cavity, axillary fossa]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Armpit \Arm"pit`\, n. [Arm + pit.]
The hollow beneath the junction of the arm and shoulder; the
axilla.
[1913 Webster]