Dialectal or informal variant of dim; somewhat dark or unclear.
From dim + -y (informal adjective suffix). This is a non-standard, regional form that appears primarily in dialectal speech rather than formal writing.
Dimmy is the kind of word kids might invent or dialects preserve—it's easier to say than 'rather dim' and it sounds casual and friendly, which is why informal speech keeps creating these kinds of variations.
Complete word intelligence in one call. Free tier — 50 lookups/day.