If he wants to file a frivolous lawsuit, he can file a frivolous lawsuit.
And that is really the whole story. Cruz was on the Hugh Hewitt talk radio program, and gave that response to a question from Hewitt whether Trump had standing to bring a lawsuit over Cruz’ eligibility to become president.
Cruz, who has impressive legal credentials, dodged [ignored, blew off, sidestepped] the substantive question of standing (that he might well have had something worthwhile to say about) and instead called Trump’s bluff. And hey, nobody likes a frivolous lawsuit.
I found out about it at Mediaite.