A saying that most teens have said at least 69 times in their lifetime. It is used to express litness. Gucci fam lit is just the perfect amount of litness its not too lit and its not under-lit either which wouldn't be good cuz u don't want yo HOTDAWG to get cold.
As others have said, used when drinking and when driving cars fast, but heavily used by motorcyclists (due to Northern Ireland having a massive motorbike culture in the 70's 80's and 90's). Keep 'er rubber side down, Keep 'er between the hedges and Keep 'er on the pipe are all other terms that spawned from Keep 'er lit in that community.