Lawful, a. — Webster enters it under “lawful”
1. Conformable to law; allowed by law; legitimate; competent.
2. Constituted or authorized by law; rightful; as, the lawful owner of lands. Lawful age, the age when the law recognizes one's right of independent action; majority; -- generally the age of twenty-one years.
Note: In some of the States, and for some purposes, a woman attains lawful age at eighteen. Abbott.
Syn. -- Legal; constitutional; allowable; regular; rightful. -- Lawful, Legal. Lawful means conformable to the principle, spirit, or essence of the law, and is applicable to moral as well as juridical law. Legal means conformable to the letter or rules of the law as it is administered in the courts; conformable to juridical law. Legal is often used as antithetical to equitable, but lawful is seldom used in that sense. -- Law"ful*ly, adv. -- Law"ful*ness, n.
Shades of meaning
Fernald has no entry of its own for lawfulness; he lists it under justice.
Where he writes about it:
Lawfulness is an ambiguous word, meaning in its narrower sense mere legality, which may be very far from justice, but in its higher sense signifying accordance with the supreme law of right, and thus including perfect justice. — from justice