"LBJ took the IRT/Down to Fourth Street/USA/When he got there/what did he see?/The youth of America on LSD." Taking "Hair" for granted as a simple, cheesy holdover from the '60s might be a mistake. Yeah, chunks of the 1967 production have a hokey, dank air about them, but, without it, theater — particularly musical theater — as we now know it simply wouldn't be the same. We all know about the infamous on-stage nudity. We know the show as the genesis of rock operas. Heck, we know the songs by heart. But the play's ascent from the 99-person capacity Joseph Papp Public Theater to Tony Awards to worldwide omnipresence and historical significance may be — other than the nude finale — the most memorable part of its legacy. The Weekend Theater's production of "Hair" runs through August 8.