Why did Wilson threaten Macomber's personal boy with lashes?

From the text:

"....and when he saw Macomber‟s personal boy looking curiously at his master while he was putting dishes on the table he snapped at him in Swahili. The boy turned away with his face blank.
“What were you telling him?” Macomber asked.
“Nothing. Told him to look alive or I‟d see he got about fifteen of the best.”