Cela est trop fort (or, raide) = That is too bad; That is beyond a joke.
Cela est par trop fort = That is really too bad.
[This par is derived from the Latin intensive particle per, as in perhorridus. In French one finds such words as parfaire, parachever, and in old French this prefix was separable. Thus, tant il est parsage might be written tant il par est sage. So, Cela est par trop fort = Cela est trop parfort.]
C’est un esprit fort = He is a freethinker.
Voilà qui est fort = That is rather strong.
Ça, ce n’est pas fort = That is very tame; There is not much in that.
A plus forte raison = All the more reason; A fortiori.
Il faut que je parle, c’est plus fort que moi = I must speak, I cannot help it.