the state of being uncircumcised; by extension, the uncircumcised taken as a people (the Gentiles)
At its root the word names a plain physical fact: the male foreskin, and so the condition of not being circumcised. It is almost unknown in everyday Greek; the Greek Old Testament used it to translate the Hebrew word for the foreskin (עׇרלה), and Paul inherited it from there.
Because circumcision marked a Jewish man as a member of God's covenant people, its absence came to stand for everything on the other side of that line. So the word slides from a bodily condition to a label for the whole non-Jewish world. When Paul sets it opposite περιτομή, "the circumcised" means the Jews and "the uncircumcised" means the Gentiles, another way of naming the nations (ἔθνος). The same word can point to a body, to a religious status, or to a group of people, depending on the sentence.
Paul's whole argument turns this loaded label inside out. He insists that the mark and its absence have stopped counting: what matters is faith working through love, not whether a person carries the sign in his flesh. The word that once sorted humanity into insiders and outsiders becomes, in his hands, a thing that "counts for nothing."