I’ve seen this gif a million times, and I just figured out the context

He’s rolling his eyes because the casting director asked him about Tallulah Bankhead, an actress with whom he had just done a play. Throughout the play’s run, Ms. Bankhead aggressively and repeatedly tried to get Brando to sleep with her, even though he found her repulsive. In a letter to his parents, he wrote “Next time she swims in the ocean, I hope whales shit on her.”

(Source: I just read his memoir)