late 14c., reformen, "to convert into or restore to another and better form" (of strength, health, firmness, etc.), from Old French reformer "rebuild, reconstruct, recreate" (12c.) and directly from Latin reformare "to form again, change, transform, alter," from re- "again" (see re-) + formare "to form" (see form (n.)).
The meaning "change (someone or something) for the better, correct, improve; bring (someone) away from an evil course of life" is recorded from late 14c.; of governments, institutions, etc., from early 15c. Intransitive sense of "abandon wrongdoing or error" is by 1580s. Related: Reformed; reforming. Reformed churches (1580s) on the European continent were usually Calvinist as opposed to Lutheran (in France they were the Huguenots). Reformed Judaism (1843) is a movement initiated in Germany by Moses Mendelssohn (1729-1786). Reform school is attested from 1859.