Archaic second-person singular past tense of 'have,' as in 'thou haddest' (you had), used in Middle and Early Modern English.
From Old English verbal conjugation patterns where '-est' was the second-person singular suffix. 'Haddest' follows the pattern of 'thou hadst' but with vowel variation typical of different dialects and time periods.
King James Bible readers would recognize 'haddest'—it's the 'thou' form that disappeared from English when we stopped using formal 'thou/thee' pronouns around the 1600s.
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