Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
dormītātor, ōris, m. [dormito], a sleeper, sluggard, commonly understood of a thief, who sleeps by day and steals by night; but better, in gen., as a dreamer, stupid fellow, only Plaut. Trin. 4, 2, 20, and 142 Brix ad loc.