I've always been very interested in words and the meanings of words, especially how they change over time. Another good example, like terrific, is Awful. Derived from the word Awe, it seems much like the word Awesome, i.e. a thing that induces awe. It actually means something negative, the exact opposite of Awesome. But it wasn't Awful that changed over the years, rather it was Awesome that changed to mean something wonderful. The original meaning of Awe in Old English was fear, terror, or dread. Thus Awful is technically unchanged.Posted by Bill Ryan (here)
I even looked up 'terrific' myself, in case Pratchett had that wrong. But he hadn't. We regard something that's 'terrific' as being extremely positive. But it originally meant 'causing terror'.
Another funny (ironic) example of changed meanings is the word silly. As far back as medieval times, 'silly' was reserved for men of the cloth, meaning 'holy' and 'pious'. But by 1500 that had changed to mean 'someone feeble of mind and lacking reason.'
If you like words and their curious and often double meanings here's a fun pic: