Albert Barnes Commentary 1 Samuel 18:25

"And Saul said, Thus shall ye say to David, The king desireth not any dowry, but a hundred foreskins of the Philistines, to be avenged of the king`s enemies. Now Saul thought to make David fall by the hand of the Philistines." — 1 Samuel 18:25 (ASV)

A hundred foreskins—This is merely another expression of the spirit which led to the constant application of the epithet “uncircumcised” to the Philistines (1 Samuel 14:6).