I've heard it many times. I searched the web and dictionary and found two meanings.
An official engaged in international negotiations (this is clear to me).
A person who deals tactfully with others (unclear).
We often say Ah! Come on...stop being a diplomat. Does it mean that we want the other person to be clear and straightforward (but then it's a bit negative) without playing with words? Does this word, by any means, refer to cunningness?
No. Being a diplomat is always neutral in American or British English, and is normally used in the literal sense that someone works as a foreign emissary on official business.
In the figurative sense, we use "being diplomatic" rather than "being a diplomat". This is used to indicate that a person is being considerate of the views of others and deals with them tactfully (which is normally a compliment, and hence used positively).
Mr Smith was very diplomatic during the meeting with union leaders.
In US English, if you hear someone say, "Stop being a diplomat!", they are telling you to quit beating around the bush or putting a positive spin on something, and say what you mean in a straightforward way, even though it may be negative.
Being "diplomatic" or "a diplomat" doesn't really refer to "cunningness", but rather to putting things in a positive light, or not assigning blame, or attempting to save face for all involved.
