To have one's back is to protect one from unanticipated matters, while one is attending to a particular function. This can be seen dramaticallyand concretely in the movie, "Gladiator." THE ACTOR
forges an alliance when he suggests that three gadiators form a team by fighting off threats from different directions (fighting back to back.) He states: "We can fight together or die separately." Also, in the
revolutionary war period, one of the notables said "We can
hang together or hang separately."
Similar
idioms would be
HAVE MY BACK= "cover me," "fight back to back," "back me up," "play
chicky for me," "keep an eye out for me," and "be ready to jump in."