Anatomy. the armpit.
Ornithology. the corresponding region under the wing of a bird.
Botany. an axil.
There is a game of croquet set up on the lawn and my second cousin Sonsoles can be found there any hour of the afternoon, bent over, with a mallet in her hand, and looking out of the corner of her eye, between the arm and the axilla, which form a sort of arch for her thoughtful gaze, at the unwary masculine visitor who appears in the harsh afternoon light.
Carlos Fuentes, “La Desdichada,” Constancia and Other Stories for Virgins, translated by Thomas Christensen, 1990
He recoiled from one odor to another until, in resignation, he accepted and his nose pumped steadily at the single generalized odor that was a meld of everything from axilla to organic debris and smelled like clam soil.
Thomas McGuane, The Sporting Club, 1968
Axilla, the Latin word for “armpit,” is a diminutive of āla “wing (of a bird or insect), fin (of a fish), armpit, flank (of an army).” Āla comes from an earlier, unrecorded ags-lā (axla in Latin orthography), one of the Latin reflexes of Proto-Indo-European ages-, aks- “pivot, pivot point.” Another Proto-Indo-European derivative, aks-lo-s, becomes ahsulaz in Germanic, eaxl in Old English, and axle in English. A third derivative noun, aks-is, becomes Latin axis “axle, axletree, chariot, wagon,” assis in Old Prussian (an extinct Baltic language), and oś in Polish. Axilla entered English in the 17th century. 🌞