c. 1300, "to administer justice;" late 14c., "to show (something) to be just or right," from Old French justifiier "submit to court proceedings" (12c.), from Late Latin iustificare "act justly toward; make just," from Latin iustificus "dealing justly, righteous," from iustus "just" (see just (adj.)) + combining form of facere "to make, to do" (from PIE root *dhe- "to set, put").
Meaning "declare to be innocent or blameless" is from 1520s. Of circumstances, "to afford justification," from 1630s. Meaning "to make exact" (now largely restricted to typesetting) is from 1550s. Related: Justified; justifier; justifying.