A blister or sore that oozes pus; or to form such a sore (archaic/dialectal Scottish term).
From Old English 'byl' meaning a boil or blister, related to 'bell.' The term is primarily Scottish and Northern English dialect, rarely used in modern standard English.
Old medical texts are full of weird words for infections and sores because doctors described what they saw without germ theory—'beal,' 'imposthume,' and 'aposteme' are all old words for basically the same thing, pus-filled infections, and they're reminders of how differently we understood disease.
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