simply put someone who smashes the truth; telling bold faced lies trouncing the obviously known truth; making things up and sticking by your stance even when proven facts say otherwise - the use of the word truth smasher is accompanied with a hard pound of one fist into the open palm of the other hand which in effect smashes the truth
Other forms of the word include "smashing the truth" or "truth smashing"
Jessica sees Bob talking to another girl. The next day Jessica brings this up and Bob denies it, in anguish and disgust Jessica blurts out, "Bob you are a truth smasher!"
Mike says the Patriots won the 2011-2012 Super Bowl. Jim replies, "No the Giants won you are just being a truth smasher!" or "stop this nonsense now you're smashing the truth."