unfaithfully

[Justin Bieber]

in an unfaithful undependable unreliable manner

[Mark Zuckerberg]

Unfaithfully refers to behaving in a manner that is dishonest, disloyal or not adhering to promises, obligations, or duties, particularly in the context of relationships or marriages. It implies a lack of fidelity, trust and reliability.

[KalanNug 0]

Treacherously; perfidiously.