English word restroom comes from English rest, English room

Detailed word origin of restroom

Dictionary entry	Language	Definition
rest	English (eng)	(obsolete) To arrest. (intransitive) To be free from that which harasses or disturbs; be quiet or still; be undisturbed.. (intransitive) To cease from action, motion, work, or performance of any kind; stop; desist; be without motion.. (intransitive) To come to a pause or an end; end.. (intransitive) To lie dormant.. (intransitive) To rely or depend on.. (intransitive) To sleep the final [...]
room	English (eng)	(dialectal, or, obsolete) Far; at a distance; wide in space or extent.. (nautical) Off from the wind. (dialectal, or, obsolete) Wide; spacious; roomy. (Internet, countable) A forum or chat room. [from 20th c.]. (always, _, in the singular) The people in a room. [from 17th c.]. (archaic) A particular portion of space. [from 11th c.]. (caving) A portion of a cave that is wider than a passage. [...]
restroom	English (eng)	(US, euphemism) A room containing a public toilet: a public lavatory.