Word of mouth  

口口相传

The movie got bad reviews, but it became popular by word of mouth.
那部电影影评不佳，但通过口口相传而变得流行起来。

Meaning / 含义：
by one person telling another; by speaking, rather than writing
指通过一个人告诉另一个人的方式传播；靠口头，而非书面形式。

Origin / 起源：
If people see something they like or don’t like, such as a movie, TV show, play, or book, and they tell their friends about it, the news is being spread by “word of mouth” instead of by television commercials, newspaper advertising, billboards, and other kinds of publicity.
如果人们看到了他们喜欢或不喜欢的东西，比如电影、电视节目、戏剧或书籍，并告诉他们的朋友，那么这个消息就是通过”口口相传”传播的，而不是通过电视广告、报纸广告、广告牌或其他形式的宣传。