Plural form or informal term, possibly referring to multiple bolls or in dialect use referring to the spherical seed pods themselves in a diminutive or affectionate way.
Likely from 'boll' with an informal or dialect plural and diminutive suffix -ie or -ies, making it a colloquial form not standard in formal English.
Regional and informal plurals like 'bollies' show how agricultural communities developed their own vocabulary shortcuts—these dialect words often disappear when farming becomes more mechanized and younger people leave rural areas.
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