屡试不爽 is formed from the characters 屡 (repeatedly), 试 (to test), 不 (not), and 爽 (to deviate). The first two characters 屡试 combine to mean 'repeatedly test,' and the last two 不爽 mean 'without error.' The phrase structures 屡 as an adverb modifying 试, and 不 negating 爽, resulting in a predicate-object construction that literally denotes repeated testing without deviation.