Source : Webster's Revised Unabridged Dictionary (1913)

Unexpressible \Un`ex*press"i*ble\, a.
   Inexpressible. --Tillotson. -- {Un`ex*press"i*bly}, adv.

Source : WordNet®

unexpressible
     adj : defying expression [syn: {inexpressible}] [ant: {expressible}]