Webster's English Dictionary

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obsess

ob.sess \*b-'ses, a:b-\ vt [L obsessus, pp. of obside-re to besiege, beset, 
   fr. ob- a]gainst + sede-re to sit - more at OB-, SIT archaic  1: HARASS, 
   BESET  2: to preoccupy intensely or abnormally