The Collaborative International Dictionary of English v.0.48:
Rereward \Rere"ward`\ (r?r"w?rd`), n. [See Rearward.] The rear guard of an army. [Obs.] [1913 Webster]Easton's 1897 Bible Dictionary:
Rereward (Josh. 6:9), the troops in the rear of an army on the march, the rear-guard. This word is a corruption of the French arriere-garde. During the wilderness march the tribe of Dan formed the rear-guard (Num. 10:25; comp. 1 Sam. 29:2; Isa. 52:12; 58:8).