To remove entirely and without remainder. A purge that leaves traces was not one.
Latin purgare (to cleanse, to purify), from purus (pure, clean) + agere (to do, to drive); a purge is the deliberate driving-out of everything unwanted, with nothing meant to remain behind.
To remove entirely and without remainder. The root's driving-out sets the bar: a purge that leaves traces was, by its own name, never actually one.