indecipherable
Concise Oxford English Dictionary © 2008 Oxford University Press:
indecipherable /ˌɪndɪˈsʌɪf(ə)rəb(ə)l/
▶adjective not able to be read or understood.
– derivatives
indecipherably adverb.
indecipherably adverb.
'indecipherable' also found in these Oxford entries:

