Simple past tense and past participle of undress.(verb)
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Use "undrest" in a sentence
"So Amyas went in, and found Frank laid on the outside of his bed not yet undrest."
"Where! almost as much terrified as the wench; while she, more than half undrest, her petticoats in her hand, unable to speak distinctly, pointed up stairs."
"While expecting his arrival, She undrest Antonia, and conveyed her to Bed."