lavatory (n.)

also bath-room, 1780, from bath + room (n.). Originally a room with apparatus for bathing (the only definition in "Century Dictionary," 1902); it came to be used 20c. in U.S. as a euphemism for a lavatory and often is noted as a word that confuses British travelers. To go to the bathroom, euphemism for "relieve oneself; urinate, defecate," is from 1920 (in a book for children), but typically is used without regard for whether an actual bathroom is involved.

1913 as a colloquial shortening of lavatory.

*leuə-, Proto-Indo-European root meaning "to wash."

It forms all or part of: ablution; alluvium; deluge; dilute; elution; lather; latrine; launder; lautitious; lavage; lavation; lavatory; lave; lavish; lotion; lye.

It is the hypothetical source of/evidence for its existence is provided by: Greek louein "to wash, bathe;" Latin lavare "to wash," luere "to wash;" Old Irish loathar "basin," Breton laouer "trough;" Old English leaþor "lather," læg "lye."