Terrific

I chose the word terrific because somehow the meaning of the word has evolved from an adjective used to describe something "dreadful, causing terror, or frightful"*.

Etymologically, the word comes from Latin, terrificus, which is composed of the stem of terrere; to frighten and the suffix ficus; causing, making.

So what occurred which made this word take on its new commonly used meaning "Very good or fine; splendid"?
The OED online provides us with some quotations which guide us through history and the words evolution.
The first definition given above was used as far back as 1667. Later, in 1809, the definition evolved to take the meaning; "Applied intensively to anything very severe or excessive". So even here the connotation has become slightly less negative.
It was only in 1930 when terrific became used as "an enthusiastic term of commendation" or "superlatively good". So now the excess defined in the second definition is being used to describe an excess of good.

So there you have it; somehow a word can be on both extremes, meaning both something which is excessively dreadful or splendid.

*All definitions/historical quotations taken from www.oed.com