The Collaborative International Dictionary of English v.0.48:

doldrums \dol"drums\ (d[o^]l"dr[u^]mz), 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.
   [1913 Webster]

   To be in the doldrums, to be in a state of listlessness
      ennui, or tedium.
      [1913 Webster]