indecipherable adj.

word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.

1520s, "find out, discover" (a sense now obsolete); 1540s, "interpret (a coded writing, etc.) by the use of a key," from de- + cipher (v.). Perhaps in part a loan-translation from French déchiffrer. From c. 1600 in the transferred sense of "discover or explain the meaning of what is difficult to understand." Sense of "succeed in reading what is written in obscure or partially obliterated characters" is by 1710. Related: Deciphered; deciphering.

1758, from un- (1) "not" + decipherable (see decipher (v.)).