Unintelligible means impossible to understand. Unintelligible speech, an unintelligible accent, largely unintelligible.
It is about the receiver as much as the sender — what is unintelligible to one listener is clear to another, which is why the word is used carefully in transcripts, where unintelligible marks a gap in the recording. Note intelligible, and that the Latin means able to be understood.
1adjectiveunintelligibleImpossible to understand.
“The announcement at the station was so crackly that it was unintelligible.”
unintelligible is a quality of obscurity.
From un- + intelligible.
Etymology from Wiktionary, CC BY-SA