1801, "spectre, apparition, ghost," from Dutch spook, from Middle Dutch spooc "spook, ghost," from a common Germanic source (German Spuk "ghost, apparition," Middle Low German spok "spook," Swedish spok "scarecrow," Norwegian spjok "ghost, specter," Danish spøg "joke"), of unknown origin. According to Klein's sources, possible outside connections include Lettish spigana "dragon, witch," spiganis "will o' the wisp," Lithuanian spingu, spingėti "to shine," Old Prussian spanksti "spark."
Meaning "undercover agent" is attested from 1942. The derogatory racial sense of "black person" is attested from 1940s, perhaps from notion of dark skin being difficult to see at night. Black pilots trained at Tuskegee Institute during World War II called themselves the Spookwaffe.
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.
"point or minute spot on a surface," Old English dott, once, "speck, head of a boil," perhaps related to Norwegian dot "lump, small knot," Dutch dot "knot, small bunch, wisp," Old High German tutta "nipple;" a word of uncertain etymology.
Known from a single source c. 1000; the word reappeared with modern meaning "mark" c. 1530; not common until 18c. Perhaps this is a different word imitative of "the mark of a mere touch with the pen" (Wedgwood). In music, the meaning "point indicating a note is to be lengthened by half" is by 1806. Morse telegraph sense is from 1838. On the dot "punctual" is 1909, in reference to a clock dial face. Dot-matrix in printing and screen display is attested by 1975.
fifteenth letter of the alphabet, from a character that in Phoenician was called 'ain (literally "eye") and represented "a very peculiar and to us unpronounceable guttural" [Century Dictionary]. The Greeks also lacked the sound, so when they adopted the Phoenician letters they arbitrarily changed O's value to a vowel. (Thus there is no grounds for the belief that the form of the letter represents the shape of the mouth in pronouncing it.) The Greeks later added a special character for "long" O (omega), and the original became "little o" (omicron).
In Middle English and later colloquial use, o or o' can be an abbreviation of on or of, and is still literary in some words (o'clock, Jack-o'-lantern, tam-o'-shanter, cat-o'-nine-tails, will-o'-the-wisp, etc.).
O' the common prefix in Irish surnames is from Irish ó, ua (Old Irish au, ui) "descendant."
The "connective" -o- is the usual connecting vowel in compounds taken or formed from Greek, where it often is the vowel in the stem. "[I]t is affixed, not only to terms of Greek origin, but also to those derived from Latin (Latin compounds of which would have been formed with the L. connecting or reduced thematic vowel, -i), especially when compounds are wanted with a sense that Latin composition, even if possible, would not warrant, but which would be authorized by the principles of Greek composition." [OED]
As "zero" in Arabic numerals it is attested from c. 1600, from the similarity of shape. Similarly the O blood type (1926) was originally "zero," denoting the absence of A and B agglutinogens.
As a gauge of track in model railroads, by 1905. For o as an interjection of fear, surprise, joy, etc., see oh.
The use of the colloquial or slang -o suffix in wino, ammo, combo, kiddo, the names of the Marx Brothers, etc., "is widespread in English-speaking countries but nowhere more so than in Australia" [OED].