A turncloak is the term used for a treacherous, disloyal person who has betrayed those who put their faith in them, those who previously considered the turncloak loyal and true and worthy of trust.
The betrayal is often an act of double-crossing, suddenly changing alliances and going over to the enemy, etcetera. To name someone a turncloak is to name them the worst kind of betrayer.
Once a person is considered a turncloak they can become a pariah and no one will put their trust in them or ever respect them, not even the people who greatly benefited from their betrayal.
- Ulf White - betrayed the blacks
- Hugh Hammer - betrayed the blacks
- Theon Greyjoy - betrayed Robb Stark for House Greyjoy
- Roose Bolton - betrayed House Stark and King Robb Stark’s cause
- Mance Rayder - betrayed the Night's Watch
- Jon Snow - Qhorin Halfhand ordered Jon to yield to the wildlings and act as a spy. Some men of the Night's Watch, such a Janos Slynt, think Jon went over willingly and they name Jon a turncloak.
- Sybell Spicer - named a schemming turncloak bitch by Ser Jaime Lannister
"Theon Turncloak," someone said as he passed. Other men turned away at the sight of him. One spat. And why not? He was the traitor who had taken Winterfell by treachery, slain his foster brothers, delivered his own people to be flayed at Moat Cailin, and given his foster sister to Lord Ramsay's bed. Roose Bolton might make use of him, but true northmen must despise him. 
"You're the one they call Turncloak. Why should we believe your promises?"