"Bad faith" is a term that refers to situations where someone acts dishonestly or deceitfully in a given situation. There are a number of synonyms for this term, including "deceitful," "disingenuous," "duplicity," "treachery," and "mendacity."
"Deceitful" refers to something that is intentionally meant to deceive, while "disingenuous" refers to someone who is pretending to be honest while actually being insincere. "Duplicity" is the act of being two-faced, while "treachery" refers to the betrayal of someone's trust. Finally, "mendacity" refers to the act of telling lies. All of these synonyms describe different facets of what can be considered "bad faith," whether intentional or not.