扪虱而谈 is an idiom
扪虱而谈mén shī ér tán
扪：按。一面捺着虱子，一面谈着。形容谈吐偷窃，无所畏忌。
Origin 《晋书·王猛传》：“桓温入关，猛被褐而诣之，一面谈当世之事，扪虱而言，旁若无人。”
Example ～，当时竟传为美事。（鲁迅《而已集·魏晋风度及文章与药及酒之关系》）
Word breakdown for “扪虱而谈”: Open in Reader →
而HSK 4top 500Among the 500 most common words 1.and2.as well as3.and so4.but (not)+4 more+5 more 谈談HSK 3top 500Among the 500 most common words surname Tan
谈談HSK 3top 500Among the 500 most common words 1.to speak2.to talk3.to converse4.to chat+1 more+2 more