the.com/backer

means A person or organization that provides financial or moral support to a project, venture, or person, especially before its success is guaranteed.

from Straightforward agent-noun territory: 'back' (as in to support, stand behind, or endorse) plus the suffix '-er' (one who does the action). The verb 'back' comes from the body part — to back someone is literally to put your back behind them, bracing them up the way you'd stand back-to-back with an ally. The financial sense of backing a venture grew naturally from this 'standing behind and supporting' meaning, well established in English by the 19th century.