From 'atsal (in its primary sense of uniting); a joint of the hand (i.e. Knuckle); also (according to some) a party-wall (Ezek. 41:8) -- (arm) hole, great.
KJV: And Ebedmelech the Ethiopian said unto Jeremiah, Put now these old cast clouts and rotten rags under thine armholes under the cords. And Jeremiah did so.
NASB: Then Ebed-melech the Ethiopian said to Jeremiah, "Now put these worn-out clothes and rags under your armpits under the ropes"; and Jeremiah did so.