Tell You The Truth Meaning

(idiomatic, informal) Used to positively assert the frank honesty of an associated statement of set of statements; equivalent to "to you tell the truth".

Example: 1994, Lawrence Block, A Long Line of Dead Men:
  "What's the point? Your buddies can kick back and relax. I'm going into voluntary retirement." / "Oh?" / "Tell you the truth, I was getting a little tired of Jim Shorter. Tired of that little room on Ninety-fourth Street. You know what I might do? I might leave town."