Etymology
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lavatory (n.)
late 14c., "washbasin," from Late Latin lavatorium "place for washing," noun use of neuter of Latin adjective lavatorius "pertaining to washing," from lavat-, past participle stem of lavare "to wash," from PIE root *leue- "to wash." Sense of "washroom" is first attested 1650s; as a euphemism for "toilet, W.C.," it is attested by 1864. Related: Lavatorial.
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lav (n.)
1913 as a colloquial shortening of lavatory.
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bathroom (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.
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*leue- 
*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."
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W.C. (n.)
"lavatory," by 1871, abbreviation of water-closet.
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loo (n.1)
"lavatory," 1940, but perhaps 1922 (based on a pun of Joyce's); perhaps [Dictionary of American Slang] from French lieux d'aisances "lavatory," literally "place of ease," picked up by British servicemen in France during World War I. Or possibly a pun on Waterloo, based on water closet.
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cloak-room (n.)

also cloakroom, 1827, "a room connected with an assembly-hall, opera-house, etc., where cloaks and other articles are temporarily deposited," from cloak (n.) + room (n.). Later extended to railway offices for temporary storage of luggage, and by mid-20c. sometimes a euphemism for "bathroom, lavatory."

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toilet (n.)

1530s, earliest in English in an obsolete sense "cover or bag for clothes," from French toilette "a cloth; a bag for clothes," diminutive of toile "cloth, net" (see toil (n.2)). Toilet acquired an association with upper class dressing by 18c., through the specific sense "a fine cloth cover on the dressing table for the articles spread upon it;" thence "the articles, collectively, used in dressing" (mirror, bottles, brushes, combs, etc.). Subsequent sense evolution in English (mostly following French uses) is to "act or process of dressing," especially the dressing and powdering of the hair (1680s); then "a dressing room" (1819), especially one with a lavatory attached; then "lavatory or porcelain plumbing fixture" (1895), an American euphemistic use.

Toilet paper is attested from 1884 (the Middle English equivalent was arse-wisp). Toilet training is recorded from 1940.

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necessary (n.)

also necessaries, mid-14c., "that which is indispensable; needed, required, or useful things; the necessities of life; actions determined by right or law; that which cannot be disregarded or omitted," perhaps from Old French necessaire (n.) "private parts, genitalia; lavatory," and directly from Latin necessarius (n.), in classical Latin "a relation, relative, kinsman; friend, client, patron;" see necessary (adj.).

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restroom (n.)

also rest room, rest-room, 1887, "room set aside for rest and quiet" (in a workplace, public building, etc.); see rest (n.1) + room (n.). As these often later had (or were required to have) accessory toilet-rooms, by 1930s the word came to be a euphemism for "lavatory, toilet."

A. ... I walked into the rest room and three or four men went in there, talking, and it seemed to me as though the place was sort of disorganized.
Q. (By Mr. CARMODY.) What do you mean by "rest room"? Do you mean the toilet?
A. That is right.
Q. It really wasn't a rest room?
A. The rest room was upstairs, over the toilet.
[NLRB vs. Pennsylvania Greyhound Lines, Inc., transcript of record, U.S. Supreme Court, October Term, 1937]
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