lying sack of shit
n. (metaphorical, yet, often simultaneously literal)  one who habitually and effortlessly utters falsehoods in a most blatant and profane manner, all the while maintaining a lackadaisical appearance of ingenuousness; despite, or perhaps, due to the fact that their entire corporeal membrane is filled to bursting with an inexhaustible supply of foul, though nitrogen-rich fecal mendacities waiting to be excreted into their victims' ears 
I can't believe you'd give any credence to anything that lying sack of shit says!

(insert name of any living politician or other person who wants something from you here) is a lying sack of shit!