armpit

n. part of the body directly below where the arm attaches to the body (Anatomy)

The underarm (or armpit, axilla, or oxter) is the area on the human body directly under the joint where the arm connects to the shoulder.
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Arm·pit n. The hollow under the upper part of the arm below the shoulder joint, bounded by the pectoralis major, the latissimus dorsi, the anterior [more]Armpit - Community and Resources

Noun
1. the hollow under the arm where it is joined to the shoulder; "they were up to their armpits in water"
(synonym) axilla, axillary cavity, axillary fossa
(hypernym) cavity, bodily cavity, cavum
(part-holonym) shoulder
(part-meronym) axillary artery, arteria axillaris

n. armpit, part of the body directly below where the arm attaches to the body (Anatomy)