mid-15c., "to keep something back, hinder," from back (adv.). Meaning "cause to move back" is from 1781. Intransitive sense "move or go back" is from late 15c. Meaning "furnish with a back or backing" is from 1728, from back (n.). Meaning "to support" (as by a bet) is attested from 1540s. Related: Backed; backing.
<a href="https://www.etymonline.com/word/backer">Etymology of backer by etymonline</a>
Harper, D. (n.d.). Etymology of backer. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/backer