You don't 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.
See also:
  • not know the half of it
  • 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