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.
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]
"toilet," 1932, probably from jakes, used for "toilet" since 15c. Meaning "prostitute's customer" is from 1911, probably from the common, and thus anonymous, name by which they identified themselves. Meaning "policeman" is by 1901, from shortening of johndarm (1823), a jocular Englishing of gendarme.
"John Darm! who's he?" "What, don't you know!
In Paris he is all the go;
Like money here,—he's every thing;
A demigod—at least a king!
You cannot fight, you cannot drink,
Nor have a spree, nor hardly think,
For fear you should create a charm,
To conjure up the fiend John Darm!
["John Darm," in "Varieties in Verse," John Ogden, London, 1823]
"latrine, outdoor toilet in a small shed, outhouse," c. 1200, from Old French privé, privee "latrine," literally "private place," from noun use of adjective privé (see privy (adj.)).
c. 1200, trone, "the seat of God or a saint in heaven;" c. 1300 as "seat occupied by a sovereign," from Old French trone (12c., Modern French trône), from Latin thronus, from Greek thronos "elevated seat, chair, throne," from suffixed form of PIE root *dher- "to hold firmly, support" (source also of Latin firmus "firm, steadfast, strong, stable," Sanskrit dharma "statute, law"). From late 14c. as a symbol of royal power. Colloquial meaning "toilet" is recorded from 1922. The classical -h- begins to appear in English from late 14c.