Two people conversing privately to the exclusion of a third
This is one of the problems that exist in social gatherings, and it is one of the ways in which Shaytaan divides the Muslims and makes some of them hate others.
The Prophet (peace and blessings of Allaah be upon him) explained the rule and the wisdom behind it: “If you are three, then two should not converse privately to the exclusion of a third until you are joined by other people, because this will make him sad.”
(Reported by al-Bukhaari, see Fath al-Baari, 11/83).
This also includes three people talking privately to the exclusion of a fourth, and so on, and two people speaking in a language which the third does not understand, because this is undoubtedly a form of disrespect towards the third, and gives the impression that they intend to hurt him, etc.