From WordNet (r) 3.0 (2006) [wn]:
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]
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]