Wiktionary
you know it
interj. (context idiomatic English) Indicates agreement, approval, encouragement.
Usage examples of "you know it".
If I find any of you not in bed in ten minutes I'll let you know it, that's all!
In London almost everything is labelled 'You mustn't touch,' and though the label is invisible, it's just as bad, because you know it's there, or if you don't you jolly soon get told.
Manfred starts making unpleasant advances, just in the line of what one could expect of such a man, and before you know it, the circus is going full blast with bleeding statues, hollowly groaning ghosts, new giant articles of clothing that appear from nowhere when you least expect it, and other merry occurrences calculated to raise the spirit of any thrill-seeking reader.