to make a shield for oneself; or, to shelter oneself from the sun and the rain with a shield of some sort (see Molina and Karttunen)
IPAspelling:
tʃiːmɑltiɑː
Alonso de Molina:
chimaltia. nino. (pret. oninochimalti.) hazer rodela para si, o guarecerse con ella del sol y del agua. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 21r. col. 1. Thanks to Joe Campbell for providing the transcription.
Frances Karttunen:
CHĪMALTIĀ vrefl,vt to shield oneself; to use something or someone as a shield / hacer rodela para si, o guarecerse con ella del sol y del agua (M), te tomo por m escudo, me sirves de escudo (C for first pers. sg. subject, second pers. sg. object) [(1)Cf.58r]. See CHĪMAL-LI. Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 52.