Whether it’s due to new technology that demands new terminology, or younger generations inventing and repurposing slang, words and their definitions change all the time. Sometimes we try using a word in a different way, or in a new context, and it ends up sticking. Ask any linguist, and they'll tell you that the definition of a word is only accurate if that's the way people are using it.
In the 14th century, the original meaning of "fantastic" was “only existing in the imagination,” as if in a fantasy. It took on the more general, superlative meaning that we are familiar with (“wonderful or very good”) in the 1930s. After the fantasy of the Roaring Twenties crashed into the Great Depression, maybe it was time to get back down to Earth.
There was once a time when vegetarians could eat meat for every meal. Although it was unlikely there were many vegetarians back then, the Middle English definition of "meat" was simply “food, nourishment, or animal feed." This included carrots, potatoes, and anything else on the menu. It wasn’t until the 1300s that the word started to refer to the “flesh of warm-blooded animals killed and used as food.”
As one of the most infamously abused words in the English language, "literally" is the bane of many grammar police. Yes, the definition since the 1530s has been “in accordance with the exact meaning of the words used.” But for over 300 years, the word has been squeezed and squished into so many dubious contexts that the Oxford English Dictionary literally added “used for emphasis while not being literally true” to the list of definitions.
Myriad used to be a word specifically for the number 10,000. This came from the Greek “myrias,” meaning the same thing. However, the word always had connotations with “a number too large to be counted,” as 10,000 was the largest number the Greeks could express in one word. Now we simply say “ten thousand,” and “myriad” means “a countless or extremely great number.”
Egregious has been flipped on its head, and the reason is ironic. In the 1500s, the word meant “excellent or distinguished.” Yet by the 1600s, egregious was being used sarcastically to mean “outstandingly bad or shocking.” This audacious use of the word is ultimately what stuck, and “egregious” has been forever tarnished.