A wasteman is a useless, unreliable or clueless person, usually a guy, the type you can't count on for anything. If it's a girl, people say 'wastegal'.
He said he'd pay me back last month and still nothing, proper wasteman.
When you're gassed, you're really excited or hyped up about something. It can also mean someone's buzzing a bit too much, like they've got carried away.
I'm so gassed for the festival this weekend, I've had my ticket for months.