the palm of the arm or hand(?) (see Molina); but, literally, the flesh of the shoulder (SW)
Alonso de Molina:
tacolnacayo. pulpejo del brazo. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 90v. col. 1. Thanks to Joe Campbell for providing the transcription.