You're having after-work drinks with coworkers and complaining about the director of your department. You've complained that he doesn't acknowledge that you're there when you see him outside of the office. Now you're continuing to talk about it, and how you think he should act. You say:
He could at least say 'hi' or something.
In this type of sentence, "could" doesn't mean "was able to" like in this sentence:
You could see Times Square from the window of his office.
Instead, "could" is used as a suggestion. It means "He should say hi."
The phrase "(someone) could at least (do something)" is used when you think someone is acting totally wrongly. In the example above, the speaker actually wants the director to be friendly and start a conversation if they see each other. But even if he doesn't act that friendly, she thinks that he should at least say "hi". So saying that someone "could at least ___" is a way to complain and tell what you think someone should do as the basic, minimum effort.
Other versions of this are:
He could say "hi" at the very least.
At least he could say "hi".
"Say 'hi'" is a common phrase to use when you're talking about greeting someone:
I was just calling to say "hi".
Say "hi" to Jeff for me, will you?
You can use "or something" at the end of your sentence to show that you're flexible, and you don't mind if something different happens.
In the example above, the speaker means that she would like for the director to say "hi", but any other kind of greeting would be OK too: smiling, waving, making small talk, and so on. Saying "hi" isn't the only action that would satisfy her.
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