1

I understand the phrase when it is used literally in this example.

He's ill and doesn't know what he's saying.

I perceive it to mean that a person is struggling with an illness that prevents him from thinking clearly and understanding his words. However, there have been times where the usage seems more vague to me, particularly when it is directed at an able-minded or sound-minded person.

He doesn't know what he's saying.

I don't think you know what you're saying.

Okay. I think there are some possibilities.

  1. A person uses the phrase as a personal attack on the other person's intellect.

  2. A person uses the phrase to discredit the other person.

  3. A person is implying that the target person is oblivious to a connotation or perception of the target person has said.

  4. A person uses the phrase to expose evidence that the target person is a dilettante.

Perhaps, the phrase means each one in different contexts, or perhaps, it means a combination of them?

2

All of those senses could indeed be meant.

The last two differ from the rest in that the rest all have in common a meaning that the statement the person made is not to be credited. So whether it's defending the speaker ("he doesn't know what he's saying, so you shouldn't be angry with him") or attacking him ("he doesn't know what he's saying; he's an ignorant blow-hard") then either way the thing that was actually said is dismissed as untrue, insignificant, or otherwise to be ignored. Indeed the emphasis is often on the fact that his statement should be discounted, than on why he made it.

Your second last example is close to that. Your last is not, and I wouldn't consider it as matching the idiom, though it's still a reasonable use of the phrase.

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  • Sounds good to me. – Double U Jan 9 '15 at 4:47
0

If his mind is working relatively normally, then we mean, at the general level, that he is not aware of the way or ways that others will interpret what he is saying.

It can be used as an attack on someone's intellect, or it can be used in support of someone. It can also be used in a way that is neutral toward him, and is focused on some fact or situation that gives his utterance a significance beyond his awareness.

There is a slight difference between the meaning of that phrase and He doesn't know what he's talking about, which means that he lacks knowledge or intelligence on his topic, as a dilettante might.

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  • I like how you included a comparison of a very similar phrase: He doesn't know what he's talking about. I would have thought they meant the same thing! – Double U Jan 9 '15 at 5:17

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