‘The dog thought this was fun, and ragrowstered with her under and on the water all the way to Leaning Willow Island.’

‘And here they left the water for a ragrowster.’

A round sound which seems to mean something like ‘roister’ or ‘roistering’.  Boisterous, playful, mischievous behaviour or tumbling.  Williamson uses it to describe otters at play (here ‘dog’ refers to a male otter), though it doesn’t appear in the OED’s 600,000 words.  Perhaps it’s a dialect word or a Williamson invention. Either way, it’s worth using and even occasionally doing.

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. 27 and 195