A turncoat witness is a witness who was expected to aid the party who called him or her to the stand, but who ended up turning into a hostile witness.
Hostile witnesses give testimonies that hurt the argument of the party who called them to the stand. So, essentially, turncoat witnesses are witnesses who unexpectedly go against their calling parties.
When a turncoat witness is cross-examined, it is as if the cross-examining party had called the witness to the stand. This is because the turncoat witness ends up helping the argument of the cross-examiner.
Lawyers usually only call witnesses to the stand who they believe will help their argument. So if a witness turns out to be a turncoat witness, it is usually not something that the lawyer could have anticipated.