“Open a can of worms,” alternatively “opening a can of worms,” is an idiom or saying that indicates that a troublesome situation has been born as a result of a remark or action. In other words, to “open a can of worms” is to unleash potential problems.

Sometimes, to indicate that a decision will lead to considerable subsequent troubles, one might say that a “dangerous can of worms has been opened,” or something to that effect. It’s often necessary to add an intensifying adjective because “opening a can of worms” is such a common idiom that it’s lost much of its force. This frequently happens when hyperbole is applied to idioms that are customarily associated with serious problems (we don’t blush when someone says that person might “bite the dust,” even though this is a fairly savage way to describe a human’s passing).

Generally, the person who “opens the can” brings trouble upon themselves, although this isn’t necessarily the case. Let us examine a couple of examples of when it might be appropriate to employ the idiom “open a can of worms.”

If you were attempting to justify communism as an economic system, you might be said to be opening a can of worms if you bring the Soviet Union’s implementation of communism into the discussion. The idiom would be applicable because you are invoking a hugely controversial and tragic era of history in bringing up the Soviet Union. You’re introducing a whole new set of problems that you have to grapple with if you invoke the Soviet Union. So, if you were attempting to argue on behalf of communism, it would likely to better to keep the Soviet experiment out of the discussion (i.e., keep that can of worms closed).

For an additional illustration, suppose that you are arguing with your spouse about financial matters, and that you point out that you believe your spouse spends money on unnecessary items, like clothes. If you also spend money on unnecessary items, you would be opening a can of worms by invoking the purchasing of unnecessary items. Why? Because your introducing an argument that doesn’t directly support your point (in fact, it may undercut it); you’re calling on something that complicates the coherence of your argument. And moreover, you may be taking the argument in a problematic direction, one that hampers the discussion (this can be another byproduct of opening a can of worms).

As you can see, “opening a can or worms” is an idiom that is particularly applicable to disputes, although this isn’t necessarily the case. In general, to “open a can of worms” is simply to take an action or make a decision that will cause trouble (broadly defined) for yourself and possibly others.