Hover over the characters to see the pinyin!

English Translation

lit.: honey in the mouth, a sword in the belly
fig.: hypocritical and murderous, with an iron hand in a velvet glove

Breaking it down

1. 口: mouth
2. 蜜: honey
3. 腹: belly (of the body)
4. 剑: sword

Source: Google Image Search

Story in Chinese

唐玄宗时, 有个大臣叫李林甫．他千方百计地收买皇帝身边的太监和宫妃来替自己说好话和探消息．皇帝喜欢什么, 他就做什么, 因此, 得到唐玄宗的信任和宠爱。
李林甫对有才能的人非常嫉妒．谁的才能、功劳超过了他, 或者谁受到了皇帝的信任, 他就要想方设法地除掉他．和人交往时, 总是装得很和善, 很谦虚, 说起话来满嘴甜言蜜语, 其实心里老盘算着害人的诡计, 所以一些人被他陷害了都没有觉察。
后来, 李林甫这虚假的面目终于被人们识破了．人们都说他是一个＂口有蜜, 腹有剑＂的人。
成语＂口蜜腹剑＂就是由＂口有蜜, 腹有剑＂简化来的．

Source: http://www.mifang.org/wx/ac/p27.html

Story in English

During the Tang Dynasty, under the rule of emperor Xuanzong, there was a minister called Li Linfu. He did everything to buy over the eunuchs and concubines around the Emperor to speak good about him.
Whatever the emperor likes, Li Linfu would do. Hence, he is trusted and favoured by his majesty.
But Linfu is very jealous of people who are talented. Whoever is more talented or skilled than him, or who earned favour of the emperor, Linfu would think of a way to get rid of him. When interacting with people, he always pretends to be very kind and humble, always speaking sweetly. But in reality, he would always be thinking of tricks to harm others.
Therefore, some people would be framed by him without even noticing it themselves.

Eventually, people saw through the false face of Linfu. They called him a "honey-mouthed but dagger-hearted" person.

Translation by Celine Li

Usage

这种人口蜜腹剑，谁知道他内心里打什么坏主意。
那个男的口蜜腹剑，几句甜言蜜语，哄得她团团转，等骗到钱后人就失踪了。