Southern English slang used to mean "Don't fuck with me" and aimed at someone who is being insulting or confrontational. Often the person who uses this word thinks he is a bad man and the person who it is aimed at has damaged their fragile ego somehow.
Chav#1: Damn man, your girlfriend has had more cocks in her than the barn her family lives in!
Chav#2: *grabs chav #1 by the collar* Listen bruv don't get lairy or i will fuck you up right here and now!!!