How "terrific" came to mean "terrific"

I've often wondered why the word "terrific" came to mean something good. "Horrific" still retains its original (unfavourable) meaning, as does "terrible". Why, then, has "terrific" come to mean "great" or "wonderful"?

Back in the day, "terrific" did in fact mean "frightening" or "terror-inducing", similar to "terrible". It was first used as such in the mid-17th century - it appears in Milton's Paradise Lost in 1667 - and was occasionally still used in this original context into the early 20th century.

In the 19th century, though, its meaning began to gradually change, through a process of semantic shift or, more specifically, amelioration, where a previously bad word takes on a good connotation ("tremendous", "awesome", "luxury" and "wicked" are a few other examples). Thus, by the early 19th century, there was already evidence of "terrific" being used to mean "severe" (as in "a terrific headache") or "very great" (as in "a terrific thunderclap"). This probably arose out of poetic exaggeration or hyperbole (i.e. so painful or so loud that it was actually terror-inducing). Gradually, it came to be used, by the 1870s and 1880s, to indicate intensity in general, and to be applied to positive experiences like "terrific beauty" or "terrific joy". From there, it is not such a great leap to its modern-day meaning.

All part of the terrific (in all its senses) English language.