a.
1.
Faithless, treacherous, perfidious, unreliable, derelict, false, deceitful, disloyal, recreant, treasonable, undutiful, false-hearted, untrustworthy.
2.
Negligent, careless, untrustworthy, unreliable.
3.
Fickle, untrue, inconstant, changeable, faithless.