Principal English Translation: 

an upper-arm bracelet or bangle, made of feathers or shells (see Molina and attestations)

Alonso de Molina: 

machoncotl. bracelete de pluma rica.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 50v. col. 2. Thanks to Joe Campbell for providing the transcription.

Attestations from sources in English: 

machoncotl = a bracelet of feathers
James Lockhart, The Nahuas after the Conquest: A Social and Cultural History of the Indians of Central Mexico, Sixteenth Through Eighteenth Centuries (Stanford: Stanford University Press, 1992), 200.

machoncotl = arm bands with quetzal feathers
James Lockhart, We People Here: Nahuatl Accounts of the Conquest of Mexico, Repertorium Columbianum v. 1 (Los Angeles: UCLA Center for Medieval and Renaissance Studies, 1993), 124.

machoncotl = the shell bracelet which the king wore on his upper arm
Mexican and Central American Antiquities, Calendar Systems, and History: Twenty-Four Papers, eds. Charles Pickering Bowditch, Eduard Seler (1904), 63.

machoncotl = upper-arm bangle
Codex Vaticanus No. 3773 (Codex Vaticanus B): An Old Mexican Pictorial Manuscript in the Vatican Library, eds. Eduard Seler and Joseph Florimond Loubat (1903), 116.

machoncotl = "the upper arms are decked with the rings extended like a shield on one side"
Codex Fejérváry-Mayer: An Old Mexican Picture Manuscript in the Liverpool Free Public Museums, eds. and transl. Eduard Seler and Augustus Henry Keane (1902), 206.

Attestations from sources in Spanish: 

machoncotl etetl = tres sonajas (Tetepango, Hidalgo, 1586)
Vidas y bienes olvidados: Testamentos indígenas novohispanos, vol. 2, Testamentos en náhuatl y castellano del siglo XVI, eds., Teresa Rojas Rabiela, Elsa Leticia Rea López, Constantino Medina Lima (Mexico: Consejo Nacional de Ciencias Tecnología, 1999), 264–265.