I believe it is the Serbs who use "May the Pope fuck your 18th-great-grandmother." (How the Pope shall choose which one of the insultee's 524,288 candidate ancestors, that's not clear.)

The famous "Go fuck yourself" directed at the Russian warship by the Ukrainians translates, literally, even better: иди нахуй (idi nahui) = "go to dick".

You make a lot of assumptions about some people's family trees. What if the 17th grandmother is also your 16th grandmother on the other side?

Eg, small village, grandmother has 6 daughters, they have 6 daughters, out of the 36 daughters 20 could be lineage grandmothers.

In fact, the further back you go, the greater chance of overlap.

(Note: growing up in a small town has no bearing on me thinking of this! None!)

the flat part is very important and makes it even more aggressive.

literally, kos chapat

the chapat is the same farsi original root word as you'd find in urdu and hindi for a chapati (the flat bread) or anything else that is flat.

Or “your asshole is a fountain” is even better.

Interesting question, and I could immediately propose:

-- that the subject ancestor availability on earth is considered a possibility, as they may be implicitly supposed to procreate at first biological occasion;

-- that the identity of said ancestor must crawl the female side of the chain, as "mater semper certa, pater numquam", especially supposedly in the case of the insultee.

Alternatively, we can make a slightly more idiomatic translation by fiddling a bit with the verb: “get on a dick” is my favourite version of this, conveying the meaning well without sacrificing too much accuracy.

(This implies that you are not mates, and may in fact be about to start having a not-good night)