Gravy (noun)

1. A sauce made from the juices of meat or vegetables that have been cooked and thickened.
2. An unexpected advantage or benefit, especially a financial one.

Origin:

Mid 16th century: from french grave, based on latin crassus 'thick'.

Examples:

1. The roast beef was served with a rich gravy made from the pan drippings.
2. The mashed potatoes were the perfect accompaniment to the gravy.
3. He worked hard to make a good living, but his investments provided the gravy on top of his salary.
4. The company's profits were good, but the real gravy came from the sales of the new product.
5. The promotion came with a pay raise and a few other perks that were the gravy on top of an already great job.