You have some magical secret that must not become known. Anyone stating this secret in your presence penalises your sorcery for the rest of the scene.
A magical secret can be almost anything. It can be nonsense, such as a particular number or the name of some higher being or your own secret name. It can be some fact about you, such as that you are a woman, your exact age or that you are the descendant of Zeus. It cannot be something people would ordinarily say; people will not accidentally blurt out your secret willy-nilly (unless the GM isusing this as a plot device).
It is possible to find your secret through research or investigation. An Investigation roll against your Sorcery skill value and some creative questions will usually do the trick, though the GM can make a montage or even scenario out of it.
Anyone who finds out your secret can use it to blackmail you and to gain dominance over you, as you cannot effectively use your powers against them. If your secret ever becomes common knowledge, you are doomed. But this should never happen if you play your cards right; the GM can invoke this as a plot device, but as long as you play along, no villain would waste the hook he has on you by publishing your secret.