Albert Barnes Commentary Jeremiah 9:26

"Egypt, and Judah, and Edom, and the children of Ammon, and Moab, and all that have the corners [of their hair] cut off, that dwell in the wilderness; for all the nations are uncircumcised, and all the house of Israel are uncircumcised in heart." — Jeremiah 9:26 (ASV)

All that are in the utmost corners — Really, all who have the corners of their hair shorn. The people meant are those Arabs who cut the hair close upon the forehead and temples, but let it grow long behind. See Leviticus 19:27.

For all these nations are uncircumcised — Or, “for all the pagan are uncircumcised.” Circumcision probably prevailed partially in the pagan mysteries as a sign of special sanctity, but to the Jews alone it represented their covenant-relation to God.