The etymology of obsession is curiously appropriate. Back in the early days of the English language, it meant "besiege", firmly reinforcing today's stalker connotations (though it also went through a brief period with supernatural connotations). This came from the French word obsession, which came from the Latin word obsessio, "a blockade". In this you can see the metynomical transition from a noun to a verb, which is quite fascinating. Anyway, obsessio is a conjugated form of the word obsidere, which is a portmanteau of the prefix ob- "against" and the root sedere "sit". This makes etymological sense since one would sit against a blockade to reinforce it. Ob- came from the PIE root opi, which doubled as "near" and "against". Sedere can also be followed back to Proto-Indo-European, in this case the word sed (which may be familiar from my post about cathedral). Thus, an obsession is to "sit nearby" your prey, a very stalker-y word back then as today.
Adam Aleksic, a freshman studying linguistics at Harvard University, has been described as the internet's sixth most famous etymologist. He has disturbing interests in words, vexillology, geography, board games, limericks, and law, and he loves writing about himself in the third person.
The Etymology Nerd