godan verb (-su), transitive verb
settle a debt; pay in full; fulfill payment obligation
Obsolete transitive verb meaning to settle a debt or pay back money/goods in full. The two senses are closely related and merged here; sense 2 is explicitly archaic. Not used in modern Japanese; replaced by 返済する or 支払う.
古文書には「借金を済す」と記されている。
In old documents, it is written 'to settle a debt'.
The historical derivation is uncertain; the kanji 済 is associated with settling or finishing, and the reading なす may be related to an older verb meaning 'to accomplish'.