A dialectal or obsolete term for a drop of liquid, or possibly a variant of 'drub' meaning to strike or beat.
Possibly related to Middle English 'droppe' (drop) or alternatively a variant of 'drub' from Old English. This is an extremely rare word with uncertain etymology, appearing sporadically in historical texts.
Some words are so obscure that even linguists aren't sure what they mean anymore—'droob' might be hiding in some 15th-century manuscript where it made perfect sense, but now we're just guessing!
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