Meaning

verb show to be reasonable or provide adequate ground for

verb show to be right by providing justification or proof

verb defend, explain, clear away, or make excuses for by reasoning

originFrom Middle English justifien, from Old French justifier, from Late Latin justificare (“make just”), from Latin justus, iustus (“just”) + ficare (“make”), from facere, equivalent to just + -ify.

Etymology from Wiktionary, CC BY-SA 3.0