Plural of cleaver: heavy blades used for cutting meat or other foods; or the climbing plant (Galium aparine) with hooked hairs that cling to clothing.
From 'cleave' (to cut) plus '-er' suffix meaning tool or device. The plant meaning comes from this 'clinging' behavior. Both uses developed in English by medieval times.
The plant called 'cleavers' is so clingy that medieval people had a clever system: they used the hooked plant as a natural strainer to filter impurities from milk, and shepherds would brush their sheep to collect the clinging stems for use as medicine.
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