Jamieson, Fausset and Brown on Jeremiah 9:26

26. Egypt--put first to degrade Judah, who, though in privileges above the Gentiles, by unfaithfulness sank below them. Egypt, too, was the power in which the Jews were so prone to trust, and by whose instigation they, as well as the other peoples specified, revolted from Babylon.

in the utmost corners--rather, "having the hair shaven (or clipped) in angles," that is, having the beard on the cheek narrowed or cut: a Canaanitish custom, forbidden to the Israelites (Le 19:27; 21:5). The Arabs are hereby referred to (compare Jr 25:23; 49:32), as the words in apposition show, "that dwell in the wilderness."

uncircumcised ... uncircumcised in the heart--The addition of "in the heart" in Israel's case marks its greater guilt in proportion to its greater privileges, as compared with the rest.