Back in the late 1960s and early 1970s, an individual who would join the "true" hippies at concerts and events, but who was not committed to the hippie cause. Plastic hippies had straight jobs, short hair and were beholden to the "man." They wanted to be a part of the hippie movement without fully committing to the lifestyle and ideals.
"The Hippie has been replaced by the pseudo-Hippie, the plastic Hippie, the weekend Hippie." 1973, Eric Berne