Source : Webster's Revised Unabridged Dictionary (1913)

Irresistible \Ir`re*sist"i*ble\, a. [Pref. ir- not + resistible:
   cf. F. irr['e]sistible.]
   That can not be successfully resisted or opposed; superior to
   opposition; resistless; overpowering; as, an irresistible
   attraction.

         An irresistible law of our nature impels us to seek
         happiness.                               --J. M. Mason.

Source : WordNet®

irresistible
     adj 1: impossible to resist; overpowering; "irresistible (or
            resistless) impulses"; "what happens when an
            irresistible force meets an immovable object?" [syn: {resistless}]
            [ant: {resistible}]
     2: overpoweringly attractive; "irresistible beauty"