Six Words That Changed Meaning

Fun was first a verb meaning "to cheat or hoax." It came from fon, an old word for "fool." It still retains some of that sense in “make fun of,” but now also means a good time.
Fond also goes back to fon, and it once meant "foolish and weak-minded." It came to then mean over-affectionate in a negative, cloying way. Now it is positive. At its root, being fond of something is basically being a fool for it.
Terrific root is terror, and it first meant terror-inducing. It then became an exaggerated intensifier (“terrifically good!” = so good it is terrifying) and then a positive term.
Tremendous has its roots in fear. Something tremendous was so terrible it caused trembling or shaking. It also became an intensifier (“tremendously good!”) before it became positive.
Awe originally referred to “immediate and active fear.” It then became associated with religious, reverential fear, and then to a feeling of being humbled at the sublime. While awful retains the negative sense, awesome took on the positive one.

To grin was to bare the teeth in a threatening display of anger or pain. It then became the term for a forced, fake smile, before settling into an expression of happiness.