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.
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.
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.
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"