(To) To support with money, influence, or encouragement: as to “back a friend.” A commercial term meaning to endorse. When a merchant backs or endorses a bill, he guarantees its value.
Falstaff says to the Prince:
“You care not who sees your back. Call you that backing of your friends? A plague upon such backing!” —Shakespeare: I Henry IV, ii.4.
“Englishmen will fight now as well as ever they did; and there is ample power to back them.” —W. Robertson: John Bright, Chap. xxxi. p. 293.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894