* Old English: The word "sad" originally came from the Old English word "sæd," which meant "sated," "full," or "satisfied." This is quite a different meaning than we associate with "sad" today!
* Shift in Meaning: Over time, the meaning of "sæd" evolved. It began to take on the sense of being "firm," "solid," or "serious." Think of how we might describe someone as "sad" today, meaning they are "serious" or "depressed."
* Modern "Sad": By the late Middle English period, "sad" had taken on its modern-day meaning of "feeling sorrow, grief, or unhappiness."
Essentially, the word "sad" underwent a semantic shift, moving from a sense of fullness to a sense of gravity and then to a sense of sorrow.
It's interesting to note that "sad" is a relatively recent word, but its origins are deeply rooted in the history of the English language.