Elon Musk stooped to a new low Sunday as he challenged Mark Zuckerberg to a "d–k measuring contest" in a bizarre tweet.

The Twitter owner, a 52-year-old adult and father of 10, made the proposition — complete with an emoji of a ruler, because of course — hours after calling the Facebook co-founder a "cuck."

The years of bitterness between the two billionaires has only intensified since Zuckerberg, 39, launched Threads as a rival to Twitter last week.

But Musk's latest posts were the final straw for some longtime Twitter users who have considered ditching the established social media platform for its promising new competitor.