The semantic hijacking of one particular word is most frequently bemoaned, particularly by our parents' generation. This word is

gay

According to Webster:
Gay a.
  1. Excited with merriment; manifesting sportiveness or delight; inspiring delight; livery; merry.
  2. Brilliant in colors; splendid; fine; richly dressed. (Why is my neighbor's wife so gay? Chaucer.)
  3. Loose; dissipated; lewd.
Webster is a little behind the times of course, and for years gay has been synonomous with homosexual.

Our generation laments the loss of that meaning, and its increasing use as a derogatory term
Dude, that is so gay