If it's physically impossible to read:
* Indecipherable: This implies the writing is illegible due to its form, like being written in an unknown script or too faded/damaged.
* Unreadable: This is a general term for anything that can't be read, regardless of the reason.
* Illegible: This specifically refers to writing that is difficult to read because of poor handwriting or messy formatting.
* Unscannable: This applies to writing that can't be read by a scanner, like a handwritten document with too much detail.
If it's conceptually impossible to read:
* Incomprehensible: This means the writing is difficult to understand, even if the words themselves are clear.
* Abstruse: This implies the writing is complex and difficult to grasp, often due to obscure language or complex ideas.
* Jargon-filled: This describes writing that uses technical or specialized language that is difficult for an outsider to understand.
* Obfuscating: This term refers to writing that is deliberately made difficult to understand, often to hide its meaning or purpose.
You could also use more descriptive phrases:
* A piece of writing that defies comprehension
* An indecipherable text
* A baffling manuscript
Ultimately, the best term depends on the specific context and the reason why the writing is impossible to read.