It's also spelled kun 君, with the same kanji as the word kimi 君, "you." Not to be confused with kun 訓, which is the kun'yomi 訓読み reading of a kanji.
Usage
The kun くん honorific is used toward boys almost the same way that chan ちゃん is used toward girls.Intimacy
The kun honorific implies an intimate, friendly relationship between the two people. Unlike chan, kun doesn't have a cutesy implication in it.Among children, boys and girls use kun くん after the name of other boys. The usage drops as the children grow up, with the the neutral san さん suffix being used instead.
Nevertheless, two good friends can refer to each other with kun くん. A girl using kun くん toward a guy can also imply she's interested in him, or at least feel they've a relationship intimate enough she doesn't need to use san さん.
Subordinates
The kun くん suffix can be used toward students, pupils, employees, and subordinates of all sorts, regardless of gender. This is a completely different usage from the usual kun くん suffix, which is normally only used toward guys.In this case, you could have a teacher calling a female student of his with the kun くん suffix, for example. Therefore, gender doesn't matter.
Since the teacher is a subordinate to the headmaster, hierarchically, the headmaster can also add the kun くん suffix to the teacher's name when referring to him. Therefore, age doesn't matter either.
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