C. F. Keil and Franz Delitzsch on 1 Samuel 18:4

1Sa 18:4

As a sign and pledge of his friendship, Jonathan gave David his clothes and his armour. Meil, the upper coat or cloak. Maddim is probably the armour coat (vid., 1Sa 17:39). This is implied in the word ועד, which is repeated three times, and by which the different arms were attached more closely to מדּיו. For the act itself, compare the exchange of armour made by Glaucus and Diomedes (Hom. Il. vi. 230). This seems to have been a common custom in very ancient times, as we meet with it also among the early Celts (see Macpherson’s Ossian).