Decipher
early 16th century: from de- (expressing reversal) + cipher1, on the pattern of French déchiffrer .
wiktionary
As decypher, but not retaining the y from the Old French etyma of cipher ( cyfre, cyffre); the i spelling tends to be preferred etymologically, being consistent with its cognates, the French déchiffrer and the Italian decifrare, and with their common ancestor, the Medieval Latin cifra, cifera, ciphra.
etymonline
decipher (v.)
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.
