turn dog. (verb) To betray someone or inform on them.

As faithful as dogs are supposed to be, they appear to be quite unreliable as well. Betraying someone seems even worse than biting the hand that feeds you; it suggests something fundamental.

This is more common in British or Australian contexts than it is in the U.S.  The meaning is well illustrated, in fact, in an Australian novel, The Landtakers.  “‘Don’t turn dog on me, Em. I stood up for you before, I’m not asking much now.’ ‘I don’t trust you an inch, Joe Gursey,’ Emma said quietly.” Without even knowing the backstory, we catch Joe’s meaning and his own backbiting personality.

turn cur. To become a police informant.