Nothing could be further from the truth
Definition 1 of 1
Pro Tip 1/3Contrast Emphasis
Used to emphasize how strongly the speaker disagrees with the previous statement or belief. 
Some say he's not a hard worker, but nothing could be further from the truth.
Pro Tip 2/3Correcting Misconceptions
Use it to correct someone's false ideas or assumptions in a strong manner. 
People assume I'm not qualified, yet nothing could be further from the truth.
Pro Tip 3/3Strong Denial
It is a way to strongly deny something and assert that the opposite is true. 
He's accused of being lazy, but nothing could be further from the truth.