Present participle of 'fest,' which is archaic or dialectal for gathering, festering, or engaging in some form of activity (highly uncommon).
Possibly related to 'fest' as a combining form from German 'Fest' (festival), or from archaic English roots meaning to gather or become infected. Usage is extremely limited and context-dependent.
This word is so obscure it's almost a linguistic ghost—it might mean gathering for a festival in one old text, or something unpleasant happening slowly in another, which is why dictionaries tend to avoid it entirely.
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