inarticulate
Meaning: incapable of expressing oneself clearly through speech
Usage: Though he spoke for over an hour, the lecturer was completely inarticulate and the students had no idea what he was talking about.
ineffable
Meaning: unspeakable, incapable of being expressed through words
Usage: It is said that the experience of playing with a dolphin is ineffable and can only be understood through direct encounter.
tacit
Meaning: expressed without words
Usage: I interpreted my parents’ refusal to talk as a tacit acceptance of my request.