This phrase references when someone needs more destruction in their relationship. The six-four comes from smashing a six-string guitar into a four-string bass guitar. The five-o references five attempts in a relationship at "getting some" and in turn getting "o" or none.
If you take your girlfriend out five times with no action, your friend could say, "Man, you need some serious 6-4 in your 5-0." Conversely, if you take your boyfriend shopping five times and he neglects each time to buy you a cinnabon, your friend could say, "Girl, you gotta get some 6-4 up in your 5-0." The form of destruction could refer to anything, not just musical instruments smashing together. For instance, you could drive your car through your girlfriend's dog's house (taking out the dog first of course). Or you could drop a road flare and some firecrackers in your boyfriend's garbage disposal.
by Ambrose Thunderfingers January 25, 2009