ecacoayo mixtli.

ecacoayo mixtli.
Principal English Translation: 

a tempestuous cloud and hurricane whirlwind (see Molina)

Alonso de Molina: 

ecacoayo mixtli. nuue tempestuosa ycon manga.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 28r. col. 1. Thanks to Joe Campbell for providing the transcription.

Attestations from sources in English: 

Looking at the elements in the expression, this might be a funnel cloud, with wind swirling in the shape of a coiled snake.