unintelligible

"Unintelligible" describes something that cannot be understood clearly. This often happens because the words themselves are hard to hear, hard to read, or badly expressed. It can also be used when an explanation is so confusing or technical that the listener or reader cannot follow it. The idea is stronger than simply saying something is difficult to understand: it suggests that the meaning is not coming through at all, or not enough to be useful.

You will often see it used for speech. If someone is mumbling, speaking through noise, or talking on a poor connection, their words may be unintelligible because you cannot make out what they are saying. It also works for writing. A note with very messy handwriting, damaged text, or too many errors may be unintelligible to the reader. More broadly, a confusing explanation full of jargon may be unintelligible to people who do not know the topic well.

The word often carries a sense of failed communication. Usually, someone has tried to understand, but the message still does not become clear. For example, a witness may give a muffled answer in court, a voicemail may be ruined by static, or a rushed speaker may sound unintelligible to the audience.

At its root, "intelligible" means able to be understood, and the prefix "un-" reverses that meaning. So if you are staring at a note you cannot read, listening to words you cannot make out, or trying to follow an explanation that makes no clear sense, this is the right word to use.