The most significant or important part of something •Usu in negative contexts: her attitude is not the half of it(1932+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with half of it
Only part of something, as in You saw them together, but that's just the half of it; she's moved in with him. This phrase, signifying the most important portion (more than half), is often put negatively as
not the half of it, as in You thought they played badly? That's not the half of it, because they've been kicked out of the league.
[ First half of 1900s