half of it - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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]