dirty, unclean, immoral, obscene
薉 centers on impurity and moral taint: something physically dirty or unclean, and by extension, behavior that is obscene or immoral.
薉 is a rare character combining 艹 (grass) with 歲, which likely contributes the sound. The exact historical development is uncertain, but it is related to the more common variant 穢.
Grass 艹 growing over an old, neglected 歲 area suggests a place that has become dirty and overgrown, reflecting impurity and moral decay.
For ワイ, imagine a dirty, overgrown yard that makes you say 'Why?' in disgust: Why -> ワイ, as you wonder why it's so unclean.