1802, from in- (1) "not" + decipherable (see decipher (v.)). Undecipherable is older. Related: Indecipherably; indecipherability.
indecipherable handwriting
an indecipherable message
indear
indearing
indebted
indecency
indecent
indecipherable
indecision
indecisive
indeclinable
indecorous
indecorum