noun, pronoun · a male sibling who is younger than oneself

我有一个亲弟弟。

noun · a humble first-person pronoun used by a male in formal correspondence

愚弟近日安好，请兄放心。

noun · a younger man or a male of lower seniority in a social or professional context

他在给前辈的信中，末尾署名为“弟”以示尊重。