1. A reference to being difficult or impossible to understand; unintelligible: The scientist used incomprehensible jargon or specialized technical terminology during his presentation to the group of chemistry students.
2. Descriptive of something which is impossible to know or to fathom because it is so complicated or mysterious: Trying to understand why the student shot so many other people in the school was incomprehensible for other students and adults.
3. Relating to that which is not understandable or decipherable: There are many incomprehensible definitions of words in some dictionaries.

Some people write with incomprehensible handwriting which makes it very difficult for others to know what is being presented and why.