copilli.

copilli = conical Huastec cap, part of the cuextecatl war costume.
Olko believes copilli is not the turquoise mosaic diadem that some have argued. That is the xiuhhuitzolli.
Justyna Olko, Turquoise Diadems and Staffs of Office: Elite Costume and Insignia of Power in Aztec and Early Colonial Mexico (Warsaw: Polish Society for Latin American Studies and Centre for Studies on the Classical Tradition, University of Warsaw, 2005), 114.

ocelocopilli = jaguar skin conical headpiece (Tezozomoc 2001, 264)
Justyna Olko, Turquoise Diadems and Staffs of Office: Elite Costume and Insignia of Power in Aztec and Early Colonial Mexico (Warsaw: Polish Society for Latin American Studies and Centre for Studies on the Classical Tradition, University of Warsaw, 2005), 159.

y iocelocupil in icpac contlaliticac = on his head he has set his conical jaguar-skin headpiece
Fray Bernardino de Sahagún, Primeros Memoriales, ed. Thelma D. Sullivan, et al. (Norman: University of Oklahoma Press, 1997), 96.

Quetzalcoatl: yn ehecatl ynteiacancauh yntlachpancauh in tlaloque, yn aoaque, yn qujqujiauhti. Auh yn jquac molhuja eheca, mjtoa: teuhtli quaqualaca, ycoioca, tetecujca, tlatlaiooa, tlatlalpitza, tlatlatzinj, motlatlaueltia. Auh yujn yn muchichioaia: ocelocopile, mjxtlilpopotz, hecanechioale, mizqujnechioale, tzicoliuhcanacoche, teucujtlaacuechcozque, quetzacoxollamamale, ocelotzitzile, icpaomjcicujle, hecacozcachimale, hecaujque, no poçulcaque. = Quetzalcoatl—he was the wind, the guide and road-sweeper of the rain gods, of the masters of the water, of those who brought rain. And when the wind rose, when the dust rumbled, and it crackled and there was a great din, and it became dark and the wind blew in many directions, and it thundered; then it was said: “{Quetzalcoatl} is wrathful.” And thus was he bedight: he had a conical ocelot-skin cap. His face was thickly smeared with soot. He was adorned with {spiral} wind and mesquite symbols. He had a curved, turquoise mosaic ear-pendant. He wore a gold neckband of small sea-shells. He had the quetzal-pheasant as a burden on his back. He had ocelot anklets with rattles. He wore a cotton bone {-ribbed} jacket. He carried the shield with the wind-shell design. He had the curved {inlaid} spear-thrower and also foam sandals. (central Mexico, sixteenth century)
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 1 -- The Gods; No. 14, Part 2, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1950), 3.