It was rock singer Mike Hanopol who introduce the word in the 70's in one of his song and it goes like this: "Laki sa layaw, laki sa layaw, Jeproks."
Literaly it means: "Grow up wild, grow up wild, Jeproks."
Jeproks was a loose translation of Hippies. It means a young man who has loose morals and easy going lifestyle, a drug addict perhaps or a a rich boy who is milking his parents wild.
Its etymology may have been the combination of the local vehicle "Jeep" and the musical genre "Rock". Jeep-rock is not a famous word but Philippine jeepneys are known to follow the career of local rock singers and turn on their radio speakers very laud to attract young passengers to ride.
I am an English Major and an etymology researcher of local vernacular. I am born in the Philippines and love my country in the same manner Jose Rizal died for her.