I’m obsessed with looking up etymologies, you know, that’s why I’m here every Tuesday! So, I figured it was appropriate that today’s word should be “obsess”!
From Latin “obsessus”, the past participle of “obsidere”, meaning to watch closely, besiege, occupy; stay, remain or abide.
The Latin word can actually be split in two: “ob”, meaning against, and “sedere”, meaning to sit, so the Latin word literally means “sit opposite to”.
When it came to English, around the year 1500, it had come to mean something like “to besiege”, and from around 1530, we get the “ghost” sense of being obsessed by an evil spirit.
The most common meaning today, though, may be the psychological one, that is “I am obsessed with FunEty!”, but that is a late sense of the word that didn’t develop until the 20th century.