An archaic spelling or variant form of dram, a small unit of weight or a measure of strong alcohol.
From dram (itself from Greek drachma, a unit of weight), this spelling is an older or regional variant. Double-m appears in historical documents.
Medieval and early modern English had wild spelling—the same word might be spelled five different ways on one page, because spelling wasn't standardized until dictionaries fixed everything in the 1700s-1800s. This is why we find dusty old texts using 'dramm' instead of 'dram.'
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