The term cowboy diplomacy is used by Jean Luc Picard to Spock as a form of an insult, in which Picard claims that Spock's attempts to discover if there may be interest in the reunification of Vulcan and Romulus represents an action that has significant consequences on the Federation.
This term is later used by Stefan DeSeve as a code word given to him by Spock, in order to prove to Picard that his request to move the Enterprise to the Kaleb sector was legitimate. (TNG: "Face of the Enemy")