This may not offer a lot of comfort, but for what it's worth - the phrase your youngest son is using is 'gangsta' langauge, and it comes from a lifestyle and music based on urban black culture, which is about as diamterically opposed to the thrash metal your older son enjoys as you can get! At five, your youngest son is not able to assimilate any influences from your older son, the phrase he has heard is something he's picked up, and does not understand. Best to ignore it, if you make it an issue, he'll do it all the more. If future disturbing phrases occur, try and find out where he heard them, without making it a major issue, and advise him calmly that such words make Mum and Dad sad, and they are not nice - that's as far as you need to go at that age.
I would be concerned that your son has friends over until four a.m., girls or boys - maybe a few house rules on entertaining guests would be a step in the right direction.