Etymology
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armpit (n.)
mid-14c., "hollow place under the shoulder," from arm (n.1) + pit (n.1). Arm-hole (early 14c.) was used in this sense but was obsolete by 18c. Another Middle English word was asselle (early 15c.), from Old French asselle, from Latin axilla. Colloquial armpit of the nation for any locale regarded as ugly and disgusting was in use by 1965.
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underarm (adj.)
1816, "underhand" (in reference to a style of throwing), from under + arm (n.1). First attested 1908 in dressmaking sense of "seams on the lower half of the arm-hole;" as a euphemism for armpit, it is attested from 1930s, popularized by advertisers.
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axillary (adj.)
"pertaining to the armpit or shoulder," 1610s, from Latin *axillaris, from axilla "armpit, upper arm, underpart of an upper wing" (see axle).
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gusset (n.)
early 14c., from Old French gosset "armhole; piece of armor for the armpit" (13c.), apparently from gousse "shell of a nut," a word of unknown origin. Originally an armorer's term; of clothing from 1560s.
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alate (adj.)
"having wings, winged," 1660s, from Latin alatus, from ala "wing, armpit, wing of an army," from *axla, originally "joint of the wing or arm;" from PIE *aks- "axis" (see axis).
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maxilla (n.)

"a jaw, a jawbone," 1670s, from Latin maxilla "upper jaw," diminutive of mala "jaw, cheekbone." "Maxilla stands to mala as axilla, 'armpit,' stands to ala 'wing'" [Klein]. Especially a bone of the upper jaw (maxilla superior) as distinguished from the mandible or lower jaw (maxilla inferior). Related: Maxillar; maxilliform.

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alar (adj.)
"wing-like," 1839; "of or pertaining to wings," 1847, from Latin alaris, from ala "wing, armpit, wing of an army" (source of Spanish ala, French aile), from *axla, originally "joint of the wing or arm;" from PIE *aks- "axis" (see axis).
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Betelgeuse 
alpha Orionis, bright reddish star in the right shoulder of Orion, 1515, from Arabic Ibt al Jauzah, traditionally said to mean "the Armpit of the Central One" (with this arm he holds his club aloft), but perhaps more accurately "Hand of al-Jauza (Orion)." Intermediary forms include Bed Elgueze, Beit Algueze.
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coxa (n.)

1706, "hip-joint," from Latin coxa "hip," which, according to de Vaan, is from PIE *koks-h- "limb, joint," and is cognate with Sanskrit kaksa-, Avestan kasa- "armpit," Old Irish coss "foot." As the first joint of the leg of an insect, crustacean or arachnid, by 1826. Cox for "thigh" was used in medical writings from c. 1400. Related: Coxalgia, coxitis.

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axle (n.)

"pole or pin upon which a wheel revolves" (properly, the round ends of the axle-tree which are inserted in the hubs or naves of the wheels), 1630s, from Middle English axel-, from some combination of Old English eax and Old Norse öxull "axis," both from Proto-Germanic *akhsulaz (source also of Old English eaxl "shoulder," oxta, ohsta "armpit," which survived as dialectal oxter; also Old Saxon ahsla, Old High German ahsala, German Achsel "shoulder"), from PIE *aks- "axis" (see axis, which is from the Latin cognate of this Germanic word). Found only in compound axle-tree before 14c.

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