Arrant means complete, total, and utter, used to emphasize something negative. When you call something arrant nonsense, you mean it's completely and thoroughly nonsensical, with not even a speck of sense in it. When someone tells an arrant lie, they're not bending the truth a little, they're fabricating the whole thing from start to finish.

The word always intensifies criticism or disapproval. You wouldn't describe someone as an arrant success or an arrant genius. Instead, you might call someone an arrant fool or arrant coward to emphasize just how foolish or cowardly they are. If a student's excuse for missing homework is arrant nonsense, the teacher sees right through it as completely made up.

Think of arrant as turning the volume all the way up on a negative description. It's similar to saying “total” or “complete,” but with more punch and usually more annoyance behind it.