Describing someone (or something) you should not trust. Break it open and it says exactly what it means: not worthy of trust — not deserving to be trusted. It is used two ways. About people, it means they may lie to you, break a promise, or let you down: an untrustworthy business partner, an untrustworthy witness. About things — information, sources, machines — it means you cannot rely on what they give you: an untrustworthy website, untrustworthy data, an untrustworthy old car. It is a serious word, stronger and colder than saying someone is 'not very reliable'. To call a person untrustworthy is close to calling them dishonest, so it lands hard; people often soften it ('I found him a little untrustworthy') or say it about a situation rather than to someone's face. Notice the shape too: unlike many un- words, it has no comfortable opposite pair in speech — people say 'I don't trust him' far more often than 'he is untrustworthy', which belongs more to reports, reviews, and careful writing.