Used by more profesional or high end prostitutes to describe there chosen line of work.

when you're hungover as shit after a spending a night drunk as a skunk

Someone who performs tasks even before he's been asked/hired/authorized to do so; he merely learns/overhears that something may need/be wished to be done, and so he himself steps in and completes said activity of his own accord, whether the people involved actually wish it to be performed or not. Just goes to show that you'll want to be really careful what you say when you're in the company of others --- you never know who might be listening, and then planning to carry out something that you merely mentioned about, not that you were actually asking anyone to perform for you.