To make a long explanation short, whiskey (with an ‘e’) refers to grain spirits distilled in Ireland and the United States. Whisky (with no ‘e’) refers to Scottish, Canadian, or Japanese grain spirits.
sauce
Fun fact! The u and simplification of words had to do with costs! In Colonial Europe, newspapers charged by the word. In the Colonial States, they charged by the letter. So if a letter wasn't really necessary, it was dropped. While it certainly doesn't explain every word, it covers the vast majority