If something is faultless, it has no mistakes — a performance, a piece of work, an accent, a plan carried out with nothing to pick at. It is a strong compliment, a little formal, and often a bit of praise with a shine on it: not merely good, but without a slip. People say it of singing, driving, manners, logic. Though actually, it is about errors, not about blame. “It was not my fault” and “a faultless copy” use fault in different rooms of the same house. Faultless does not mean “nobody is to blame”; it means “no flaw showed.” In real talk, we also use it with a small stretch: a “faultless” evening may still have been lucky, not perfect in every tiny way.