"You can't be too careful when a young child is near water."
Collins Dictionary (https://www.collinsdictionary.com/dictionary/english/you-cant-be-too-careful)
I've understood from the dictionary definition that the adjective phrase "too careful" in the idiom You can't be too careful means, basically, "careless". In other words, I understand the idiom like this: You can't be careless when a young child is near water.
My question is: isn't "(too)careful" an antonym of "careless"?
No. You can't be too careful means that it is impossible for you to be excessively careful. The basic idea is that it is OK to be very, very, very, very.
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No.
You can't be too careful means that it is impossible for you to be excessively careful.
The basic idea is that it is OK to be very, very, very, very. very careful.