Archaic second person singular present tense of 'be' (thou abbest = you are), used in early English texts following Latin grammar patterns in ecclesiastical language.
From Old English 'bēon' with the archaic '-est' second person singular suffix. Though unusual, it appears in some Early Modern English texts influenced by Latin ecclesiastical writing, where religious terminology sometimes forced unusual grammatical constructions.
Words like 'abbest' are linguistic fossils—they show how English monks and clerics sometimes tried to force English grammar to follow Latin patterns because so much theology was written in Latin; 'abbest' appears in a handful of medieval religious texts and then vanishes forever.
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