something half way, half full, at the half way point (see Molina); presumably, "tlaco" (half) is at work here
Orthographic Variants:
tlatlacouilli
IPAspelling:
tɬɑtɬɑhkowiːlli
Alonso de Molina:
tlatlacouilli. cosa demediada. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 137v. col. 1. Thanks to Joe Campbell for providing the transcription.