The term PET ROCK, ONCE is a reference to a FAD from the 1970s. The three letter solution for this term is FAD, which stands for 'FAD' or 'FADDISH'. A FAD is defined as a fashion or practice that is very popular for a short period of time. The Pet Rock was a popular toy in the 1970s that consisted of a rock in a cardboard box with air holes and a manual. It was marketed as a low-maintenance pet and became a popular novelty item.