Definition: to make a speech or written opinion weaker and less controversial.
(This phrasal verb has more than one meaning)
E.g.1. A: I found some of his views rather extreme.
B: And I heard that he watered down that version of the speech so that it would appeal to more people!
E.g.2. 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.
This phrasal verb can be separated.