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 leading contender for valedictorian of his high school, is a 208-month-old boy with disturbing interests in etymology, vexillology, geography, and law. Adam would like to one day visit Tajikistan and just won an essay contest on linguistics!
The Etymology Nerd