illegible

Illegible means written so badly that it cannot be read. The letters are there and nobody can make them out.

It describes handwriting, signatures and worn inscriptions, and doctors' handwriting is the traditional joke. It is worth keeping apart from unreadable, which usually means too dull or difficult to get through rather than physically impossible to decipher. Its opposite is legible, which simply means readable. The related noun illegibility is rare, and most people say the writing is illegible instead.