noun

• (archaic or dialectal) Sorrow; repentance; regret.
  "When I was one-and-twenty I heard him say again, "The heart out of the bosom was never given in vain; 'Tis paid with sighs aplenty and sold for endless rue." And I am two-and-twenty, and oh, 'tis true, 'tis true."
• (archaic or dialectal) Pity; compassion.

verb

• (transitive) To repent of or regret (some past action or event); to wish that a past action or event had not taken place.
  "to rue the day"
• (obsolete, transitive) To cause to repent of sin or regret some past action.
• (obsolete, transitive) To cause to feel sorrow or pity.
• (archaic or obsolete, intransitive, often with on or upon) To feel compassion or pity; to take pity (on), to have compassion (on).
  "[…] till our Lord's clouds rue upon the earth, and send down a watring of rain: Truly , I think Christ's misty dew a welcome message from heaven, till my Lord's rain fall : […]"