Source : Webster's Revised Unabridged Dictionary (1913)

Doldrums \Dol"drums\, n. pl. [Cf. Gael. doltrum grief,
   vexation?]
   A part of the ocean near the equator, abounding in calms,
   squalls, and light, baffling winds, which sometimes prevent
   all progress for weeks; -- so called by sailors.

   {To be in the doldrums}, to be in a state of listlessness
      ennui, or tedium.