Source : Webster's Revised Unabridged Dictionary (1913)

Armpit \Arm"pit`\, n. [Arm + pit.]
   The hollow beneath the junction of the arm and shoulder; the
   axilla.

Source : WordNet®

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}]