The definition of indecipherable is something that cannot be read or understood.
Really messy handwriting that you cannot read is an example of something that would be described as indecipherable.
that cannot be deciphered; illegible
Impossible to decipher: indecipherable handwriting; an indecipherable message.
- in′de·ci′pher·a·bil′i·ty in′de·ci′pher·a·ble·ness
(comparative more indecipherable, superlative most indecipherable)