not know the half of it

not know 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 don't know 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 don't know the half of it."
See also: half, know, not, of
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