þeostru f (-e/-a) darkness, gloom (both literally and figuratively). Has a constellation of related words surrounding it, including þeostrig adj(obscure, blinded); þeostrian wv/t2 (to make dark or dim); þeostorloca m (tomb or dark enclosure);

I'll be brewing a Münchner Dunkel tomorrow and I think I'll name it Theosterness. Google returns about 270 hits for "theost" with most of them being misspellings of "the lost" or "the most" but there was one that seemed to use the word in a sense consonant with the meaning of the Old English word.