"No sweat" is an idiomatic expression that means something is easy or effortless. There are several antonyms for this phrase including "difficult," "challenging," "complicated," "arduous," and "troublesome." These words denote a sense of exertion, struggle or difficulty in completing a task. They are used to describe circumstances that require effort, time, or resources to overcome. For example, a math problem or a complex assignment might be categorized as "difficult," while a bad day at work may be considered "challenging." Depending on the situation, using an antonym for "no sweat" can help to convey a sense of difficulty or challenge that might otherwise be overlooked.