Source : Webster's Revised Unabridged Dictionary (1913)

Irrepressible \Ir`re*press"i*ble\, a.
   Not capable of being repressed, restrained, or controlled;
   as, irrepressible joy; an irrepressible conflict. --W. H.
   Steward.

Source : WordNet®

irrepressible
     adj : impossible to repress or control; "an irrepressible
           chatterbox"; "uncontrollable laughter" [syn: {uncontrollable}]