Its untrue and derogatory. While both primates, humans don't have any monkeys as ancestors. The term monkey is applied to humans when someone wants to belittle their behavior, often when we want to point out that one group is lesser than another group who does not behave that way.

It's true and derogatory. But it's just derogatory for humans who have an exaggerated view on themselves. Stanford's Robert Sapolsky had an excellent course on Behavioural Biology. You can watch it for free.

While monkeys and hominids (apes, humans & chimpanzees) are both primates, they are separate groups that have evolved separately. Any inherent behavior we share with monkeys either came from a common ancestor or was co-evolution.

It is derogatory because it is used to belittle and dehumanize. It has and is commonly used by people with an exaggerated view on themselves to slur other groups, most famously against Africans. Monkey behavior is assumed to be lower and less desirable, and something that should be overridden in humans or be corrected for. Correctly defining things as human behavior (even if primitive behavior shared with our ancestors) is neutral, identifying it as natural and default behavior inherent to our species, and not a joke or slur.

Yes, the Sapolsky lectures are excellent and still hold up IMO.

Sorry, I confused monkeys and apes. In german, we have no nice single-term for monkeys, just Affen for Primates and Menschenaffen for Hominidae, and I didn't thought about that before.

But it makes not much difference. A friend of my wife once gifted her a capuchin monkey, so I could observe the astonishing behavioural similarities to humans (human children) first hand. Since then, I see more and more of them, especially in group behaviour.

And, btw, I don't give a damn how other people use the terms.