obsess

ob•sess v.

to dominate the thoughts of;
preoccupy:[~ + object]Revenge obsessed him.
to think about something without stopping:[no object]He obsessed about his old girlfriend for years.

ob•sess v.t.

to dominate or preoccupy the thoughts, feelings, or desires of (a person);
beset, trouble, or haunt persistently or abnormally:Suspicion obsessed him.

v.i.

to think about something unceasingly or persistently;
dwell obsessively upon something.

Latin obsessus, past participle of obsidēre to occupy, frequent, besiege, equivalent. to ob- ob- + -sid(ēre) combining form of sedēre to sit
1495–1505

ob•sess′ing•ly, adv.
ob•ses′sor, n.

Collins Concise English Dictionary © HarperCollins Publishers::

obsess /əbˈsɛs/ vb

(transitive; when passive, followed by with or by) to preoccupy completely; haunt
(intransitive; usually followed by on or over) to worry neurotically or obsessively; brood

Etymology: 16th Century: from Latin obsessus besieged, past participle of obsidēre, from ob- in front of + sedēre to sit

'obsess' also found in these entries (note: many are not synonyms or translations):

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