liar is not believed when he tells the truth
liar is not believed (even) when he tells the truth
Prov. If people think that you are a liar, they will not believe anything you say. As it turned out, Fred was right when he warned his friends that the police were planning to raid their party; but they paid no attention to him, since they knew him to be a liar, and a liar is not believed even when he tells the truth.