The German word for “trustworthy”, in the sense of a person who is honest and truthful, is “vertrauenswürdig” (literally “trustworthy”). In the sense of “reliable”, the word is “zuverlässig”, and in the sense of “credible” or “believable”, the German word is “glaubwürdig” (literally “belief-worthy”). The opposites are formed either by attaching “un” to the beginning of the word, as in English, or by using the word “nicht” (“not”) just prior. The nouns are formed by adding “keit” to the words above (always feminine):

– “Ich finde diesen Mann nicht besonders vertrauenswürdig.” – “I don’t find this man (to be) particularly trustworthy.”
– “Sie macht einen vertrauenswürdigen Eindruck.” – “She makes a trustworthy impression.”
– “Diese Automarke ist nicht sehr zuverlässig.” – “This brand of car is not very reliable.”
– “Deine Ausrede ist nicht glaubwürdig.” – “Your excuse is not believable.”
– “Du machst dich unglaubwürdig.” – literally “You’re making yourself untrustworthy.”
– “Du verlierst an Glaubwürdigkeit.” – “You’re losing credibility.”
– “Zuverlässigkeit ist sehr wichtig.” – “Reliability is very important.”

Relatedly, as when applying for a loan, the word for “creditable” or “creditworthy” is “kreditwürdig”.