"can't underestimate" = can't overestimate

[Laurence Horn]

Much discussion on Language Log.  But I've always thought this one is more complex than, say, "No head injury is too trivial to ignore", simply because of the different meanings available for "can't".  "You can't overestimate it" means it's so important that it's impossible to overestimate; "You can't underestimate it" as you say approaches "shouldn't", but "can't" often *does* mean "shouldn't" (or "mustn't"),  in deontic contexts where the meaning is 'it would be a mistake to' ("You can't/mustn't/shouldn't stay out late").  Both "can't overestimate" and "can't underestimate" make sense in the context on different readings of "can't".

LH

On Oct 18, 2011, at 9:09 AM, Jonathan Lighter wrote:

> A CNN analyst says "You can't underestimate" the Israelis' relief at
> the release of Gilad Shalit.
>
> I've heard this quasi-reversed construction frequently. It seems to
> confuse "can't" and "shouldn't."
>
> JL
>
> --
> "If the truth is half as bad as I think it is, you can't handle the truth."
>
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