Armpits

The hollow spaces under the upper arm where it joins the shoulder and torso. Plural of 'armpit.'

A compound word combining 'arm' (from Old English 'earm') and 'pit' (from Old English 'pytt' meaning 'hole'). The term has been used since at least the 1600s, though the concept of a body cavity has existed in anatomical language much longer. 'Pit' is a common suffix for body cavities, as in 'hip pit' (though less common now).