something flat and even, such as a board or the ground
Alonso de Molina:
cemixmanqui. cosa llana e ygual, como tabla, o suelo. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 16r. col. 1. Thanks to Joe Campbell for providing the transcription.