something flat and even, such as a board or the ground
IPAspelling:
semmɑnki
Alonso de Molina:
cemmanqui. lo mesmo es que cemixmanqui. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 16r. col. 2. Thanks to Joe Campbell for providing the transcription.