have not heard the half of it

have not heard the half of it

To know 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."
See also: half, have, hear, not, of