to make a speech or written opinion weaker and less controversial. (This phrasal verb has more than one meaning and it can be separated)
- I found some of his views rather extreme.
- And I heard that he watered down that version of the speech so that it would appeal to more people!
- I prefer to read strident, forceful opinions that I can disagree with; the articles in that newspaper have all been watered down and checked for political correctness.
Source: Knights English