Source : Webster's Revised Unabridged Dictionary (1913)

Obsess \Ob*sess"\, v. t. [L. obsessus, p. p. of obsidere to
   besiege; ob (see {Ob-}) + sedere to sit.]
   To besiege; to beset. --Sir T. Elyot.

Source : WordNet®

obsess
     v 1: haunt like a ghost; pursue; "Fear of illness haunts her"
          [syn: {haunt}, {ghost}]
     2: be preoccupied with something; "She is obsessing over her
        weight"