Meaning of "naturam unibilitatis"

Question

In Summa theologiae (ST I q. 29 a. 1 ad 5) one can read:

Ad quintum dicendum quod anima est pars humanae speciei, et ideo, licet sit separata, quia tamen retinet naturam unibilitatis, non potest dici substantia individua quae est hypostasis vel substantia prima; sicut nec manus, nec quaecumque alia partium hominis. Et sic non competit ei neque definitio personae, neque nomen.

(emphasis mine)

English translation ("Benziger Bros. edition, 1947") reads:

Reply to Objection 5: The soul is a part of the human species; and so, although it may exist in a separate state, yet since it ever retains its nature of unibility, it cannot be called an individual substance, which is the hypostasis or first substance, as neither can the hand nor any other part of man; thus neither the definition nor the name of person belongs to it.

(emphasis mine)

However word unibility seems to be absent from at least some online dictionaries.

One can find some explanation of concept of unibilitas on philosophical or maybe theological level in Francesco Bottin's "Unibilitas. Back to the source of the soul's unibility to the body" where following can be found:

That which specifically differentiates human souls from angelic essences is precisely unibilitas, i.e. the inclination to unite with a body which only characterizes the human souls in an essential way.

(emphasis mine)

This provides some insight into the meaning of this phrase.

Therefore it can be asked:

what does naturam unibilitatis mean?

(the phrase is in accusative, so it should be natura in nominative, but how to change unibilitatis?)

An attempt would be "nature of inclination to unite with something/of inclination to union"

Answer 1

It seems to me like you answer your own question. The word is quite precise and certainly not going to be found in classical dictionaries, but specialist dictionaries contain the word.

The Thomas-Lexikon (unfortunately in German!) translates unibilitas as "Vereinbarkeit," which is roughly "compatibility."

Such a translation illuminates the meaning but perhaps obscures the fact that Thomas means no more nor less than the most obvious (and equally awkward) translation: "unibility," i.e. the ability to be united.

Aquinas uses the term 17 times, according to the Index Thomisticus (searching for "=unibilitas") and in almost every occasion it refers to the soul's disposition to be the "form" of a body. According to this view, adopted from Aristotelian hylomorphism, there are two principles in composite entities: form and matter. In the case of living beings, the form is the soul and the matter is the body. "Unibilitas" refers to the fact that the soul contains within itself a disposition to be united to the body as its form. I suspect that Aquinas uses "unibilitas" here instead of "unio" or "coniunctio" because, in his view, the human soul, being immaterial, does not need to be the form of the body but can exist separately.

The literal and accurate translation, in my opinion, would simply be nature of unibility. A freer but still accurate gloss would be, natural disposition to union [with the body].

Answer 2

naturam unibilitatis

naturam is accusative, as you said, but unibilitatis is genitive, thus the 1947 literal translation "nature of inability" is precisely correct.