A highly obnoxious person who never shuts up about Jesus, God, church, or any combination of these. They constantly try to convert people to their religion, and often mistake politeness as an agreement to join their cultish Jesus-freak ways.
A common trait in Jesus Freaks is their total lack of interest in actually following the morals and such of their own religion, even though they pretend to (i.e., lying, adultery, etc). They also believe that Jesus and God make everything better, and anything negative in their lives must be pinned on Satan.
Of course, they only use God and Jesus and religion as a crutch because they can't handle the harsh reality that is called life.
Jesusfreak: I was reborn in Christ, and now everything is magically better!
Not-Jesusfreak: ...I don't believe in God, jesusfreak.
Jesusfreal: Get thee away, SATAN!
(Bible verse-quoting and holy water-throwing ensues.)