In The Sentence, The Wind's Like A Whetted Knife. What Does The Word Whetted Mean?

'Whetted' means 'sharpened'. So, a whetted knife would mean a sharpened knife. This sentence therefore means that the wind was blowing so fiercely (or perhaps it was so cold) that it felt very sharp. It does not mean that the wind will injure you the way a sharp knife would, but it is used metaphorically to convey how sharply the wind is blowing.

You can also alternatively say that the wind was biting.

The word 'whetted' may be used for other tools that may be sharpened too (not in this case though). Another common usage of the word is 'whetted appetite'. When you become very hungry, your appetite is whetted. Just like grinding sharpens a knife, an aroma or ambiance may sharpen the appetite:

The aroma of fresh bread wafting through the windows of his cottage whetted the farmer's appetite.