It means that when a person is lying to you - and doing it so badly that their disingenuous worthless words are an insult to your intelligence.
What is harder is a liar who is comfortable telling lies with nonchalance & ease. Fortunately there are facial ticks that give them away but only is 80% accurate on the general popluation. The better you know the person the better it is to spot a lie.
Learning the 'tells' works good for playing poker or when negotiating across the table.
How to spot a lie : - https://hubpages.com/relationships/BTLS
People Manipulators - http://hubpages.com/t/14c4d5