What does the phrase holier than thou mean?

offensively self-righteous

Like I'm better then you..

I read in Matthew 6:17 when Jesus said, "but thou, when thou fastest, anoint thine head and wash thy face.

A true definition of this saying (You think you're holier than thou) is (you think you're holier than you, because when ever the Word thou was used it was translated from the Word you. "But you, when you fast."