When two separate aspects or relationships in a persons life
collide: your friends (one "world") become friends with your girlfriend (a separate "world").
Popularized by The Seinfeld episode "the pool guy", in which
Georges Costanzas' girlfriend forms her own friendship with Georges'
platonic friend Elaine (Causing Georges' worlds to collide).
Kramer: 'Jerry, don't you see? This world here, this is George's sanctuary. If Susan comes into contact with this world, his world's collide. You know what happens then?'
KRAMER: 'Ka shha shha shha Pkooo!'