"Two or more people saying the same thing."
Explanation here,
http://www.talktomeinkorean.com/shows/ctw0510/
It's not about lips getting together and kissing, which could be misinterpreted as... Haha
2015年1月19日
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Sorry..'about' should be changed into 'that'.
2015年1月19日
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Here ~라고 입을 모이다 means to have the same discussion about~.