not faithful; false to duty, obligation, or promises; faithless; disloyal.

not sexually faithful to a spouse or lover.

not accurate or complete; inexact

an unfaithful translation.

Obsolete . unbelieving; infidel.

noun

disloyalty , false-heartedness , falseness , falsity , infidelity , perfidiousness , perfidy , traitorousness , treacherousness , treachery , apostasy , bad faith , defection , dereliction , fickleness , improbity , inconstancy , recreancy , treason

adjective

errant , unfaithful , wayward