Whitewash has two distinct meanings:

1. A white liquid paint made from lime and water, used to quickly and cheaply paint walls, fences, and buildings. Farmers traditionally used whitewash on barn walls because it was inexpensive and helped keep buildings cooler in summer. The most famous literary whitewashing happens in Mark Twain's Tom Sawyer, where Tom cleverly convinces his friends that painting a fence with whitewash is so enjoyable that they should pay him for the privilege of doing his chore.
2. To cover up or hide mistakes, wrongdoing, or unpleasant facts to make something look better than it really is. When a company tries to whitewash a scandal, they might downplay what happened or leave out important details. A student might whitewash a story about breaking a window by conveniently forgetting to mention they were throwing rocks. A history book might whitewash the past by presenting only the positive aspects of historical events while ignoring the troubling parts.

This second meaning connects to the first: just as literal whitewash covers a dirty surface with fresh white paint, figurative whitewashing covers up uncomfortable truths with a cleaner, more acceptable version of events. When someone accuses another of whitewashing, they're saying, “You're not telling the whole truth.”