Impossible to break especially under ordinary usage.

Roget 327: tenacious, tough, strong, resisting, sequacious^†, stringy, gristly cartilaginous, leathery, coriaceous^†, tough as whitleather^†; stubborn etc. (obstinate) 606.    unbreakable, indivisible; atomic.    ... show more

Dutch: onbreekbaar
Polish: niełamliwy