Underarm (noun)

1. The inner side of the arm where it joins the shoulder.
2. The area under the arm, especially the armpit.

Origin:

Late middle english: from under + arm.

Examples:

1. She raised her underarm to sniff for any body odor.
2. The underarm of the shirt was stained with sweat.
3. He shaved his underarm hair before the beach trip.
4. The underarm area was sore from the injection.
5. She applied deodorant to her underarm before heading out.