Etymologically speaking, the stupefying source of sad is very satisfying! The word traces back to Old English saed, which meant "fulfilled or satisfied". As weird as this seems, it actually kind of makes sense. One who is satisfied, say by food, will enjoy that comfortable feeling of drowsiness after a good meal. Thus sad started to mean "weary and tired", and since people with depression or great sadness often feel weary, the word came to be applied to the emotion they were experiencing. This is a rare and scintillating instance of a complete about-face in etymology, something pretty scarce in the field. Anyway, saed is generally acknowledged to trace back to the Proto-Germanic word sathaz (or sadaz; this d to th distinction is common in Germanic tongues, and is represented by the symbol ð, which I will avoid to prevent confusion). Sathaz traces to its reconstructed etymon from Proto-Indo-European, seh, both of which meant "satisfied". Seh , incidentally is also the root of Latin satis, which gave today's word, satisfied. This is why linguistics are amazing! You'll never expect these completely shocking origins!
Adam Aleksic has been described as the internet's sixth most famous etymologist. He loves writing about himself in the third person, he's a freshman at Harvard University, and he has disturbing interests in linguistics, vexillology, geography, board games, limericks, and law.
The Etymology Nerd