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.
Hello! I'm Adam Aleksic, a senior studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy trivia, politics, vexillology, geography, board games, conlanging, art history, and law.