Both terrible and terrific go back, through French and Latin to Proto Indo European roots meaning to shiver. The origin is self explanatory: extreme fear being well known to provoke shivering.

To these two you can therefore add tremor and tremble.

In English "terrible" and "terrific" are antonyms. Terrific means exceptionally good and on the contrary, terrible means exceptionally bad.

In French however, terrific does not exist and terrible has both significations. Only the context, common sense or the status of the speaker will tell you which meaning to choose from.

If one says "c'est pas terrible", that will mean below average. If a youngster says "this singer is terrible", that has to be understood as appreciative.

In "Ivan le terrible" though, the meaning is obviously not "the Great".