Where a thing came from. Naming the source is what makes a claim checkable rather than merely asserted.
Old French sourse (a spring, a rising), from sourdre (to rise up), from Latin surgere (to rise); a source was first a spring of water rising from the ground, before it named anything a claim could be traced back to.
Where a thing can be traced back to — naming it is what turns a claim into something checkable rather than something merely asserted.