As much as a bat can hold or carry; the amount that fills one bat or container.
From 'bat' (a container or stick) plus the suffix '-ful' meaning 'full of' or 'the capacity to hold.' Creates a unit of measurement.
Medieval measurements were wonderfully creative—a batful of grain or flour was literally what you could carry in one bat-shaped container, meaning measurements varied by person and purpose until standardization arrived.
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