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
Farlex Dictionary of Idioms.

You don't know the half of it.

Inf. You really don't know how bad it is.; You might think that what you have heard is bad, but you do not know the whole story. Mary: They say you've been having a bad time at home. Sally: You don't know the half of it. Sally: The company has no cash, they are losing orders right and left, and the comptroller is cooking the books. Mary: Sounds bad. Sally: You don't know the half of it.
See also: half, know, of
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

You don’t know the half of it

sent. Things are far more complicated than you think. You don’t know the half of it, and I’m too much a lady to tell.
See also: half, know, of
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • have not heard the half of it
  • as bad as all that
  • have a bad opinion of (someone or something)
  • have a good, bad, high, low, etc. opinion of somebody/something
  • be downhill all the way
  • downhill all the way
  • it's the story of my life
  • story of my life, the
  • (it's/that's the) story of my life
  • bad company