Not easily deciphered; difficult to read(adjective)
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Use "undecipherable" in a sentence
"I stole a glance at Patch, to see his reaction to what I’d just done, but his eyes were on the poker game, his expression undecipherable."
"The Oriental words and phrases are, in several instances, very incorrectly printed; but whether the fault rests with the colonel's "undecipherable" MS., or the correctors of the press, it is not for us to decide."
"He didn’t use any unnecessary words, and his expression was undecipherable."