Principal English Translation:
measurement from an elbow to the other hand (see Molina)
Alonso de Molina:
cemmitl. medida desdel vn codo hasta la otra mano.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 16v. col. 1. Thanks to Joe Campbell for providing the transcription.
Attestations from sources in English:
Graphically, this appears on maps as an arrow. See the Oztoticpac Lands Map.
Víctor M. Castillo F., "Unidades nahuas de medida," Estudios de Cultura Náhuatl 10 (1972), 195–223, see especially page 206, fig. 4, and 209, fig. 5.
Attestations from sources in Spanish:
"Por lo tanto, puede considerarse el cémmitl equivalente a 1.25 m; es decir a un medio del cémmatl."
Víctor M. Castillo F., "Unidades nahuas de medida," Estudios de Cultura Náhuatl 10 (1972), 217.