c. 1500, "to besiege" (a sense now obsolete), from Latin obsessus, past participle of obsidere "watch closely; besiege, occupy; stay, remain, abide" literally "sit opposite to," from ob "against" (see ob-) + sedere "to sit," from PIE root *sed- (1) "to sit." Of evil spirits, "to haunt," from 1530s. The psychological senseof "to haunt as a fixed idea" developed gradually from 1880s and emerged 20c. The 1895 Century Dictionary has only the two senses "to besiege" (marked obsolete) and "to attack, vex, or plague from without." Related: Obsessed; obsessing.
