Best believe
Example:
Person 1: I heard Rosa said she was going to fight you
Person 2: if she touches me best believe I will whip her ass
Person 1: I heard Rosa said she was going to fight you
Person 2: if she touches me best believe I will whip her ass
you best believe
you best believe means that you better believe it, i’m not lying. if you say “you best believe” you are obviously telling the truth