Any amount of money earned above and beyond the expected. Examples: finding a $20 on the ground, winning money while gambling, buying something cheap and selling it at a higher price.
Money earned at work as part of salary or normal hourly wages would not be considered walking slicks. However, any additional money earned, such as a bonus, would be considered walking slicks.
12 step recovery jargon referring to someone new who talks about how great life is, now that they'resober. Usually meaning that the person is out of touch with reality.
The new guy seems pretty happy for a dude who has no job, no money and no family. He must be on a pink cloud.