Would you please tell me what this word means? I heard it in Lily Allen's 'Alfie', as a way to address her little brother, in the song.
It's French. It means "my brother."
Many English-speakers would understand. It's sometimes used for comedic emphasis when disagreeing with someone. You could interject a French "Au contraire!" but it's funnier (cause it rhymes) to say "Au contraire, mon frere!"
In your example, it's probably used as a term of endearment.