murky

The Streets of London, to be beheld in the very height of their glory, should be seen on a dark, dull, murky, winter's night, when there is just enough damp gently stealing down to make the pavement greasy without cleansing it of any of its impurities, […]

Ferdinand: As I hope / For quite dayes, faire Iſſue, and long life, / With ſuch loue, as 'tis now the murkieſt den, / The moſt opportune place, the ſtrongſt ſuggeſtion, / Our worſer Genius can, shall neuer melt / Mine honor into luſt,[…]

They may face an impenetrable word, approach its murky history from every direction, and fail to find a convincing solution (or even any solution: “origin unknow,” “the rest is unclear,” and the like).