adverb/entɾeˈdjentes/
under one's breath; in a murmur; quietly; especially, so as to avoid being overheard, as when saying something disrespectful
“El padre de Selena dijo entredientes, pero determinado, "¡Yo no me siento con esa gente!"” — Selena's father said under his breath, but determinedly, "I won't sit with those people!"
From entre (“between”) + dientes (“teeth”).