We don't have synonyms for dimmit yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are dimwit, dammit and immit.
(n) :
(UK dialectal, Devon, Cornwall, West Country) Alternative form of dimmet (Twilight; dusk; crepusculum) [(UK dialectal, West Country) Twilight; dusk; crepusculum.]
(n) :
(UK dialectal, Devon, Cornwall, West Country) Alternative form of dimmet (Twilight; dusk; crepusculum) [(UK dialectal, West Country) Twilight; dusk; crepusculum.]