When you're good in bed. You got them off and actually did your part unlike someone who just laid there. A guy might call you good in the sack if you were kinky and into trying things as well.
Getting fired from your job. Australians use this phrase a lot. Probably from the pioneer days when the boss would throw a bag at the farmhand/miner/builder/labourer's head, tell him to pack his stuff and get lost.