In common usage, "terrific" is one of those words in the English language that is paradoxically its own antonym. It can mean "very good" or "very bad." For example, you can say "I've got a terrific headache" or "I had a terrific vacation." Note that neither of these usages corresponds to its literal meaning.

See Words That Are Their Own Antonyms.
This information shamelessly ripped-off from Car Talk

Ter*rif"ic (?), a. [L. terrificus; fr. terrere to frighten + facere to make. See Terror, and Fact.]

Causing terror; adapted to excite great fear or dread; terrible; as, a terrific form; a terrific sight.

 

© Webster 1913.

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