What’s the catch is an idiom that dates to the 1800s. We will examine the meaning of the idiomatic phrase what’s the catch?, where it came from, and some examples of its use in sentences.
The phrase what’s the catch? is a retort when one believes that an offer is too good to be true. One may ask what’s the catch? if he believes that a situation or deal is being presented on its best face and that problems or drawbacks are being hidden. For instance, if a stranger approaches you and hands you a hundred dollar bill, you may wonder what you will have to do to repay that stranger, or if the money is stolen. In this case you may respond, “What’s the catch?” The word catch, in this case, means a hidden cost or a hidden trap. The idiom what’s the catch? came into use in the 1850s and is associated with P. T. Barnum, a flamboyant American showman who is associated with hucksterism and skirting the truth.
So the U.S. Court of Appeals for the 11th Circuit ruled it was unconstitutional to force felons to pay all their financial obligations before allowing them to register to vote, but what’s the catch? (The Tampa Bay Times)
After a big snowfall, roads in Chugiak-Eagle River are cleared faster and cheaper than those in Anchorage. So what’s the catch? (The Chugiak-Eagle River Star)
Given that Google is all about the data, by which I mean your data, what’s the catch? (Forbes Magazine)