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.
Friend: "Last night I won $1000 at the casino"
Me: "Oooohh, nice... walkin slicks"
Friend (Nodding): "Yeeaaahhhhh..."