Archaic second-person singular form of 'do,' as in 'thou doyst,' used in Early Modern English (roughly 1500-1700).
From Old English 'dōn' with the suffix '-est' added for second-person singular conjugation. The 'y' is a scribal convention from Early Modern English printing to distinguish words and fill space.
If Shakespeare wrote 'thou doyst,' we'd read it as 'you do'—but the 'y' isn't a vowel here, it's a printer's filler! Early typesetters would add random letters to make lines look even and justified.
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