from back (n.) or (adv.)
Perhaps originally a friend who ‘kept back,’ and did not come forward to assist, and so was no real friend.
From: Dramatic Works of Philip Massinger
Volume the First
Edited by Thomas Coxeter, 1761
The Virgin-Martyr. A Tragedy.
By Philip Massinger, and Thomas Decker
Act II, Scene I. P. 23