Why did 'nice' once mean 'foolish', and 'awful' once mean 'impressive'?

Words drift. A word has several senses and connotations, and they can be added, removed or altered over time, sometimes so far that the modern meaning is nothing like the original. Linguists call this semantic change, and it happens to words you use every day.

Take 'nice'. It originally meant 'foolish, ignorant, frivolous, senseless'. It comes from Old French nice, from the Latin nescius, 'ignorant or unaware'. Literally, it meant 'not-knowing'. From there its senses wandered: 'timid, faint-hearted' before 1300, then 'fussy', then 'dainty', then 'precise, careful', which survives in 'a nice distinction'. Only in 1769 did it reach 'agreeable, delightful', and in 1830 'kind, thoughtful'.

Sometimes a word splits and the halves run in opposite directions. 'Awful' literally meant 'full of awe' and originally 'inspiring wonder (or fear)', hence 'impressive'. Today it means 'extremely bad'. 'Awesome' started from the same place and now means 'extremely good'. 'Terrible' and 'terrific' did the same: both began with 'inspiring terror', moved to anything spectacular, and then one went to spectacularly bad and the other to spectacularly good.

The word 'guy' has a vivid origin story. Guy Fawkes was the alleged leader of a plot to blow up the English Houses of Parliament on 5 November 1605. A ragged figure of him was burned on the holiday and called a 'Guy', which led to the word meaning any person of grotesque appearance. By the late 1800s, especially in the United States, it meant any man.