In a way that breaks a promise — most often the promise between two people who are together. If someone "behaves unfaithfully," they had sex with, or a romance with, someone who is not their husband, wife, or partner. The word is an adverb, so it tells you HOW someone acted: "He acted unfaithfully." Honestly, this is not a common word in daily speech. English speakers almost never say "he behaved unfaithfully" out loud — they say "he cheated on her," or "he was unfaithful." You will meet unfaithfully in writing: newspapers, novels, court reports, church talk, anything careful or old-fashioned. It sounds serious and a little distant, which is sometimes exactly why a writer picks it — it does not sound as angry as "cheated." There is a second, older use, outside of love: to do a job unfaithfully means to do it without keeping your promise to it — a servant, a translator, a person paid to guard something. You will see this mostly in old books. Both uses share one idea: a promise was made, and the person did not keep it.

不忠地、出軌地（副詞）