If someone or something hadn't helped, intervened, or affected the situation in some way. The phrase is always followed by an explanation of how things would have turned out differently without someone or something. If it hadn't been for those quick-thinking bystanders, I wouldn't be alive today. If it hadn't been for those meddling kids, we would have gotten away with it.

To know only a small part of how bad something is, but without understanding the complete situation; to underestimate or be uninformed about how bad a situation really is. Our bosses think that this is a major setback, but they haven't heard the half of it. We're months, not weeks, behind schedule! A: "I heard you and Terry were going through a tough time." B: "Oh, you haven't heard the half of it."

To not have very much intelligence or common sense, especially when facing or presented with a difficult situation. I'm not surprised his latest business venture failed too—that fool doesn't know enough to come in out of the rain. Cut him some slack, Jim. You didn't know enough to come in out of the rain when you were his age, either.

slang Should not have. I hadn't oughta stayed out so late last night—I'm exhausted. You hadn't oughta said that to her because now she's mad.