The rereward (see Joshua 5:9). Literally, the gathering together and then the body of troops which collects the stragglers, the rear guard, as in Numbers 10:25; Isaiah 52:12; Isaiah 58:8. Calvin renders here by quia cogebat agmen.
But the LXX. and Vulgate render by ὁ λοιπὸς ὄχλος and vulgus reliquum. So Luther, der Haufe. The LXX; however, in Isaiah 58:9 translates the same word by οὐραγοῦντες, i.e; "qui extremum agmen ducunt, et quasi caudam efficiunt" (Rosenmuller).
The word is not the same as that translated rereward in 1 Samuel 29:2, the only other place where our version has "rereward," where there can be no question of the rendering being correct, since the literal meaning there is the hindermost.