Faithless means disloyal or unable to be trusted to keep promises or commitments. A faithless friend might share your secrets with others or abandon you when you need help. A faithless employee might steal from their company or secretly work for a competitor.

The word carries a sense of broken trust. When someone is faithless, they've failed to honor an agreement, a relationship, or a responsibility they were supposed to uphold. A faithless teammate might not show up for practice or might deliberately play poorly to help the other side win.

Faithless can also describe someone without religious faith or belief, though this meaning is less common today. More often, we use it to describe people who betray others' confidence in them.

The opposite of faithless is faithful, meaning loyal and trustworthy. A faithful dog stays by your side. A faithful friend keeps their word. Being faithful means others can count on you to do what you promised, while being faithless means you've let them down by breaking that trust. Once you've acted faithless, rebuilding trust becomes incredibly difficult, since people naturally hesitate to believe promises that have already been broken.