You Know It Meaning

(idiomatic) Indicates agreement, approval, encouragement.

Example:   "What he did to her was wrong." "You know it, sister!"
2004, Tom Perrotta, Little Children, page 301
  "I've still got a few years on the chain gang." "Counting the days, huh?" "You know it."
2006, Dorothea Benton Frank, Full of Grace, page 19
  He's a little bit older, mind you, but good people, if you know what I'm saying to you …" "Italian, right?" "You know it, baby, and —" "Over fifty? Widowed? Grown children?" "You're a mind reader!"
2007, Sheila M. Goss, Roses Are Thorns, Violets Are True, page 96
  "Man, I wonder if David found out. Is that why they divorced?" "You know it."