An English south-west dialect word for dusk or twilight, hinted at in the opening ‘dim…’.
‘At dimmity it flew down the right bank of the river …’
‘At dimmit light …’
Not to be confused with ‘dimity’, a strong cotton fabric woven with stripes or checks.
A surely related word, with the same meaning, also from the English south-west, is ‘dimpsey’, with ‘dimpsy’ as adjective.
‘At dimpsey, when day and night hunters see each other between two lights…’
‘It’s getting a bit dimpsy’.
There must be a ditty in there somewhere, what with dimmit dimmity dimpsey dimpsy.
Source: Henry Williamson, Tarka the Otter: His joyful water-life and death in the two rivers, illus. C.F. Tunnicliffe (Harmondsworth: Puffin Books, 1976 (1927)), p. 21, 48 and 45, and OED