From Gibbon, Ch. XVIII (The time is the triune principate of Constantine II, Constantius and Constans, and the topic is Sapor, the Persian monarch).

But as soon as Sapor attained the age of manhood the presumptuous Thair {a king of Yemen or Arabia}, his nation, and his country, fell beneath the first effort of the young warrior, who used his victory with so judicious a mixture of rigour and clemency that he obtained from the fears and gratitude of the Arabs the title of Dhoulacnaf , or protector of the nation. 

In my Everyman edition, Oliphant Smeaton (ah, they don’t name them like that any more!) comments thus:

Sir John Malcolm in his History of Persia has stated that Gibbon has made an error in the derivation of the name “Dhoulacnaf”. It means Zoulaktaf, or Lord of the Shoulders, from his directing the shoulders of his captives to be pierced and then dislocated by a string passed through them.

Fascinating title, but can anyone cast further light on the Persian words involved?

The term seems to be Arabic in origin, and was used to refer to Shapur II of the Sassanians, who fought against the Khazars (an ostensibly Jewish tribe from the Caucasus) and nomadic Arabs in the 6th C. CE.  He’s fondly remembered for drilling holes in the shoulders of Arab prisoners in order to pass a rope through them to keep them from flight.  Here’s a quote below:

As the Khazars, like the Huns, were a branch of Ural Altay tribe, so they surrounded the Sasanians region in question. In order to hinder their invasion, Anushiravan created Caucasus Canyon. The invasions and inroads of nomad Arabs were also among the difficulties which had to be faced, because their styles, which were based on murder and plunder, were somewhat different from those of the Romans. To hamper and suppress the invasion of desert dwelling Arabs, effective steps were taken by Emperor Ardeshir I and Emperor Shapur II by Arabs known as “Zolaktaf” and Emperor Khosrow Anushiravan; but the Sasanians through confirming and supporting the Emir of Hireh in Khvārvarān province (what is today Iraq), in practice charged him with the task of blocking the invasion of the desert dwelling Arabs.

The spelling is generally Zolaktaf, and is also, interestingly, a reasonably common surname in present-day Iran.

Dhu (a)l- is a common Arabic phrase meaning ‘master/owner of the’; Alexander the Great is known in Arab folklore as Dhu ‘l-Qarnayn ‘Master of the Horns.’ Gotta go eat dinner; maybe afterwards I’ll look up the second parts of the names in my Arabic dictionary.

Ah, so they’re Arabic. Which is the closer transliteration to the actual sound, Dhou- or Zou-? I assume then that cnaf = country and ktaf = shoulders?

I’ve seen it translated as Lord of the Shoulder, and From the Shoulder; my own suspicion—and anyone please correct me if I’m wrong—is that it might be closer to ‘from the shoulder’.  That is, Zol - ak - naf (?), with ak being the active verb.  Just a wild guess, and I’m interested to find out how close (or, more than likely, how far off) I am.  In terms of pronunciation, well, that’s a tough one.  We have little clue as to how Bede or Alfred may have spoken, so to get a reasonably proximate fix on this one?  I’ll leave that to the professional forensic linguists.  Like any part of the world at that time, spoken or written language was far from standardization; present-day speakers might help us, but who really knows?

Oops—that spelling is Zolaktaf.

Which is the closer transliteration to the actual sound, Dhou- or Zou-?

Depends.  In most of the Arabic-speaking world, the /dh/ sound (as in English those) has disappeared, replaced by either /z/ or /d/, so it would be pronounced /zu-/.  In Iraq and parts of the Arabian Peninsula, though, the /dh/ is preserved.

I assume then that cnaf = country and ktaf = shoulders?

Actually, checking with my Arabic dictionary I find that kanaf, plural aknāf, is ‘side, flank; wing; shadow, shelter; bosom’; kat(i)f, plural aktāf, is ‘shoulder; (mountain) slope; buttress.’ So Dhu’l-Aknāf would be ‘Lord of the Sides/Shelters’ and Dhu’l-Aktāf ‘Lord of the Shoulders/Slopes’; I don’t think it’s possible to know which is correct.