'backward-flowing' originates from Modern English, specifically formed by combining the adjective 'backward' and the present participle 'flowing'.

'backward' developed from Middle English 'bakward' (from Old English 'bæcweard', itself from 'bæc' meaning 'back' + suffix '-weard' meaning 'toward'); 'flowing' is the present participle of 'flow', from Old English 'flowan' (from Proto-Germanic *flōaną). The compound is a straightforward Modern English formation combining those elements.

Initially, 'backward' meant 'toward the back' and 'flow' meant 'to move along a surface or through a channel'; combined as 'backward-flowing' the phrase has the clear, literal meaning 'flowing in a backward/reverse direction', a usage that is largely compositional rather than having developed a distant figurative sense.