Someone gave me an unsatisfactory answer as to the etymology of this name, so I did some research, and believe I've figured it out myself.
There are 7,000 people in France with that surname, and about 700 apiece in Russia and Romania. As a first name, it is enormously popular in that last location with well over 70,000 examples.
Anyone can give their child any first name, but surnames are much more stable. Since the name's connection to that last location appears no earlier than about 1852 with the publishing of Dimitrie Bolintineanu's poem Sorin sau taierea boierilor la Targoviste (Sorin, or the cutting of the boyars at Targoviste), after which it became extremely popular in that country, we should start with the surname in France.
This means we should look at similar names in France, in particular those with the same suffix -in, such as Colin and Robin. And indeed these are perfect examples, Colin a compound of the 'nickname' in that country at the time for Nicolas, with the diminutive suffix -in, which can indicate a possessive, such as 'son of', and Robin, a compound of the 'nickname' for Robert, with the same suffix.
Sorin in France then appears to come from the old name Sorrel (also Sorel) based on an old word for "red" or "chestnut" color, and used as a name for someone with such characteristics, hair color for example. While it is somewhat more notable as a girl's name nowadays, it was not always that way historically, and still has a sizable representation even today as a name for boys as well. Sorin then, would be an example of shortening of that name, which Robin and Colin are both similar examples of, with the addition of the diminutive suffix -in. The surnames Hugon and Huggins have a similar etymology, the first part from Hugo and/or Hugh, with the same form of suffix. There are many examples, too numerous to recount here.
As to the Russian incidence of the name, this comes from the Ashkenazic Jewish girl's name Sore (Sarah) with the Slavic suffix -in or -kin, such as Sorkin (the inheritance of Judaism is passed along the matrilineal line, and so the matriarch may be afforded recognition through the family name in many cases).
The Romanian incidence can be chalked up to the nationalist poet Dimitrie Bolintineanu's use of the name in his almost 'call-to-arms' level poem published during the ethnogenesis of that country, and that nation-state's historic starry-eyed obsession with all things France, from where the name originates. The use of the name in conjunction with the nationalist poem containing "Cutting of the Boyars in Targoviste" in the title would then be an obvious example of poetic use since the name means 'son of the red haired man'. Dimitrie Bolintineanu was studying in Paris from 1845-1851 according to various sources (see below).
https://forebears.io/surnames/sorel
https://forebears.io/surnames/sorrel
https://en.wikipedia.org/wiki/Colin_(given_name)
https://forebears.io/surnames/colin
https://en.wikipedia.org/wiki/Robin_(name)
https://www.ancestry.com/name-origin?surname=colin
https://en.wiktionary.org/wiki/sor#Middle_English
https://www.ancestry.com/name-origin?surname=sorin
https://www.ancestry.com/name-origin?surname=sorkin
https://forebears.io/surnames/sorin
https://forebears.io/forenames/sorin
Published 4/23/23