The original meaning of 진짜 is "genuine ". antonym is 가짜(fake) The original meaning of 정말 is "real story". antonym is 거짓말(lie) Two words can be used as meaning of "really". There is no difference between 정말 and 진짜 in that meaning. 그래 is a short form of "그러해". It is from "그러하다" 그러하다 means "like that" 그래 literally means "It is like that" and somewhat similar to "Yes, OK" 그래? literally means "Is it like that?" and somewhat similar to "Really?"