The name Fortinbras derives from French, and the original pronunciation would have been something like 'fortinbra'h'. In English, however (most prominently as the name of a character in Shakespeare's Hamlet) the final 's' sound is usually pronounced.
2
That is, 'strong in the arm' is the literal meaning of the name Fortinbras. In the case of Fortinbras II, the choice of name was doubtless influenced by his great-grandfather Fortinbras Took I, the father of Gerontius the Old Took.