Albert Barnes Commentary 1 Samuel 27:10

"And Achish said, Against whom have ye made a raid to-day? And David said, Against the South of Judah, and against the South of the Jerahmeelites, and against the South of the Kenites." — 1 Samuel 27:10 (ASV)

The Jerahmeelites - That is, the descendants of Jerahmeel, the son of Hezron, the son of Perez, the son of Judah (see marginal references). They were therefore a portion of the “south of Judah.”

The Kenites - See the note on Numbers 24:21; Judges 4:11; and for their close proximity to Amalek, see 1 Samuel 15:6.