Traditionally, used to denote the beginning of (American) baseball season practice anxiously awaited by enthusiasts each springtime: If there was grass growing, then the season had progressed enou gh past winter to start the new season: "play ball!" More recently, it has become a slang inferring statutory laws regarding underage females did not, in practice,
realistically apply and that instead, the presence of
pubic hair (grass) indicated sufficient maturation to begin
sexual activity (play ball).