The full saying is "No one ever went broke underestimating the intelligence of the American people/public," or some variation thereof, meaning that people being swindled won't realize or question it, which makes them a prime source of income for others. It is typically attributed to writer H.L. Mencken. A: "We can't sell this shoddy product to people!" B: "Oh please, no one ever went broke underestimating the intelligence of the American people."