Used to emphasize that you are telling the truth

Congratulations. This was a tricky question and you gave a very good answer, I kid you not!

Complimenti! Era una domanda insidiosa e tu hai risposto molto bene. Dico sul serio!

The famous and honorable preacher was found in "intimate" behavior with a trans and he justified himself saying that he was trying to redeem and save him. I kid you not!

Il famoso e integerrimo predicatore è stato trovato con un transessuale in "intima" compagnia e si è giustificato dicendo che lo faceva per poterlo redimere. Non sto scherzando!

This catchphrase was popularized by Jack Paar, host of the Tonight Show from 1957 to 1962, who used it as his signature phrase