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).