undress (v.)
1590s, "to shed one's clothing," from un- (2) "opposite of" + dress (v.). Transitive sense of "to strip off (someone's) clothing" is recorded from 1610s. Related: Undressed; undressing.
undress (n.)
"state of partial or incomplete dress," 1680s, from undress (v.). Meaning "ordinary dress" is from 1748.