c.1300, "clear of a charge or suspicion;" late 14c., "cleanse, clear, purify," from Anglo-French purger, Old French purgier "wash, clean; refine, purify" morally or physically (12c., Modern French purger) and directly from Latin purgare "cleanse, make clean; purify," especially of the body, "free from what is superfluous; remove, clear away," figuratively "refute, justify, vindicate" (also source of Spanish purgar, Italian purgare), from Old Latin purigare, from purus "pure" (see pure) + root of agere "to drive, make" (see act, n.). Related: Purged; purging.

1560s, "that which purges," from purge, v.. Meaning "a purgative, an act of purging" is from 1590s. Political sense from 1730. Earliest sense in English was the now-obsolete one "examination in a legal court" (mid-15c.).