noun

• (nautical) The vertical edge of a sail that is closest to the direction of the wind.
  "By easing the halyard, the luff of the sail was made to sag to leeward."
• (nautical) The act of sailing a ship close to the wind.
• (nautical) The roundest part of a ship's bow.
• (nautical) The forward or weather leech of a sail, especially of the jib, spanker, and other fore-and-aft sails.

verb

• (nautical, of a sail, intransitive) To shake due to being trimmed improperly.
  "I thought how my life is like a little boat and I must hold the tiller steady against the buffeting of wind and waves, and how sometimes, like this morning, I lose my hold somehow and the sail luffs helplessly and the little vessel wallows, turning this way and that in the swell."
• (nautical, of sailing vessels, intransitive) To bring the ship's head up closer to the wind. (Alternatively luff up)
  ""Helm there! Luff, luff a point! So; steady, man, steady!""
• (nautical, transitive) to let out (a sail) so that it luffs.
• (mechanical) To alter the vertical angle of the jib of a crane so as to bring it level with the load.
  "The tower is mounted on a slewing platform, which also carries the power plant and the counterweights, while the jib is supported and luffed by fixed pendant ropes."