What is the X in X-mas?
c.1500, "to besiege," from Latin obsessus, past participle of obsidere "watch closely; besiege, occupy; stay, remain, abide" literally "sit opposite to," from ob "against" (see ob-) + sedere "sit" (see sedentary). Of evil spirits, "to haunt," from 1530s. Psychological sense is 20c. Related: Obsessed; obsessing.