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I usually interpret it as "I couldn't agree more."

Right, you only use it when the person has completely captured your ideas or feelings on the subject, and you have nothing left to add.

Then add nothing. Your agreement isn't valuable information.

That's the OP's point, yes, but parent comment asked for a definition.

Also "I was going to say this... but this guy beat me to it."

"The post is word-for-for the one that I opened this page in order to post".

Agreement.