General terms:
* Inarticulate: Unable to express oneself clearly or effectively.
* Unintelligible: Not able to be understood, especially in speech.
* Mumbling: Speaking in a low, unclear voice.
* Slurring: Speaking with a distorted pronunciation of words, often due to intoxication or a speech impediment.
* Stuttering: Repeating sounds or words, or having difficulty starting to speak.
Medical terms:
* Aphasia: A language disorder that affects a person's ability to communicate. This can include problems with speaking, writing, reading, and understanding language.
* Dysarthria: Difficulty speaking clearly due to problems with muscle control in the mouth, face, or vocal cords.
Other terms:
* Mute: Unable to speak at all.
* Nonverbal: Not using language or speech to communicate.
The best way to describe someone who cannot speak understandably depends on the specific situation and the reason for their difficulty speaking.
It's important to be respectful and avoid using language that is stigmatizing or offensive.