The Collaborative International Dictionary of English v.0.48:

Brood \Brood\ (br[=o]ch), v. i. [imp. & p. p. Brooded; p. pr.
   & vb. n. Brooding.]
   1. To sit on and cover eggs, as a fowl, for the purpose of
      warming them and hatching the young; or to sit over and
      cover young, as a hen her chickens, in order to warm and
      protect them; hence, to sit quietly, as if brooding.
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            Birds of calm sir brooding on the charmed wave.
                                                  --Milton.
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   2. To have the mind dwell continuously or moodily on a
      subject; to think long and anxiously; to be in a state of
      gloomy, serious thought; -- usually followed by over or
      on; as, to brood over misfortunes.
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            Brooding on unprofitable gold.        --Dryden.
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            Brooding over all these matters, the mother felt
            like one who has evoked a spirit.     --Hawthorne.
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            When with downcast eyes we muse and brood.
                                                  --Tennyson.
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