Q: Why do people say we “can’t underestimate” something significant when they mean we “can’t overestimate” it? People seem to take a liking to such a mistake and never listen to themselves to question whether it makes any sense.
A: You’re right. People do say we can’t underestimate something when in fact they mean we can and do underestimate it!
But would it be more accurate to say, as you suggest, we “can’t overestimate” it? The answer isn’t quite as simple as you think.
The linguist Mark Liberman has written an interesting post on the Language Log about whether the usage you object to is indeed wacky, and why it’s so persistent.
If “can’t underestimate” is illogical, Liberman would argue, it’s because our “poor monkey brains” have a hard time dealing with complex statements, especially those involving more than one negative idea, or because English simply has a lot of nutty idioms (like “I could care less”).
But on the other hand, perhaps it’s not so illogical after all. Liberman also discusses the argument that this usage makes perfect sense if “can’t” is defined loosely as “may not” or “must not” or “should not.”
In other words, when people say we “can’t underestimate” something, what they may really mean is that we “shouldn’t underestimate” it.
Although many usage experts frown on the looser uses of “can,” especially as a substitute for “may,” this usage is common in speech and informal writing. For example, when we say, “Hey, you can’t do that,” we mean it’s not allowed.