the doorjamb or pivothole below the door (see Molina; transl. to English here by SW)
Orthographic Variants:
tlatzacuillotl ycayan
Alonso de Molina:
tlatzacuillotl ycayan. el quicio baxo dela puerta. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 142v. col. 1. Thanks to Joe Campbell for providing the transcription.