the.com/burden of proof

means The obligation to provide evidence for a claim rests with the person making it, not with others to disprove it.

from A legal phrase translating the Latin 'onus probandi' — 'onus' meaning load or burden, as in cargo weighing down a cart. Roman and later common-law courts used it to assign who must carry the evidentiary weight in a dispute: the accuser, not the accused, hauls the load. The metaphor stuck because it's apt — proof really is treated as freight that someone has to lug into court, and whoever makes the claim is stuck carrying it.