Insuppressible

It seemed as if the sense of public resentment had long been gathering strength unperceived, and now burst forth into insuppressible violence.

Source: wiktionary

In its stead Victor favoured Karslake with a slow smile of understanding that broadened into an insuppressible grin of successful malice, a grimace of crude exultation through which peered out the impish savage mutinously imprisoned within a flimsy husk of modern manner.

Source: wiktionary