c. 1200, "mild, well-mannered," Old English gedæfte "gentle, becoming," from Proto-Germanic *gadaftjaz (source also of Old English daeftan "to put in order, arrange," gedafen "suitable;" Gothic gadaban "to be fit"), from *dab-, which has no certain IE etymology and is perhaps a substratum word.

Sense deteriorated to "dull, awkward, uncouth, boorish" (c. 1300), perhaps via the notion of "humble." Further evolution to "foolish, simple, stupid" (mid-15c.) and "crazy" (1530s) probably was influenced by analogy with daffe "halfwit, fool, idiot" (see daffy); the whole group probably has a common origin. For sense evolution, compare nice, silly. Related: Daftly; daftness.

late 14c., "lucky, favored by fortune, being in advantageous circumstances, prosperous;" of events, "turning out well," from hap (n.) "chance, fortune" + -y (2). Sense of "very glad" first recorded late 14c. Meaning "greatly pleased and content" is from 1520s. Old English had eadig (from ead "wealth, riches") and gesælig, which has become silly. Old English bliðe "happy" survives as blithe. From Greek to Irish, a great majority of the European words for "happy" at first meant "lucky." An exception is Welsh, where the word used first meant "wise."

Happy medium "the golden mean" is from 1702. Happy ending in the literary sense recorded from 1756. Happy as a clam (1630s) was originally happy as a clam in the mud at high tide, when it can't be dug up and eaten. Happy hunting ground, the reputed Native American paradise, is attested from 1840, American English. Happy day for "wedding day" is by 1739; happy hour for "early evening period of discount drinks and free hors-d'oeuvres at a bar" is by 1961, said to be 1950s. Rock-happy (1945) was U.S. Pacific theater armed forces slang for "mentally unhinged after too much time on one island." Related: Happier; happiest.

Happy family an assemblage of animals of diverse habits and propensities living amicably, or at least quietly, together in one cage. [Century Dictionary]

late 13c., "foolish, ignorant, frivolous, senseless," from Old French nice (12c.) "careless, clumsy; weak; poor, needy; simple, stupid, silly, foolish," from Latin nescius "ignorant, unaware," literally "not-knowing," from ne- "not" (from PIE root *ne- "not") + stem of scire "to know" (see science). "The sense development has been extraordinary, even for an adj." [Weekley] — from "timid, faint-hearted" (pre-1300); to "fussy, fastidious" (late 14c.); to "dainty, delicate" (c. 1400); to "precise, careful" (1500s, preserved in such terms as a nice distinction and nice and early); to "agreeable, delightful" (1769); to "kind, thoughtful" (1830).

In many examples from the 16th and 17th centuries it is difficult to say in what particular sense the writer intended it to be taken. [OED]

By 1926, it was said to be "too great a favorite with the ladies, who have charmed out of it all its individuality and converted it into a mere diffuser of vague and mild agreeableness." [Fowler]

"I am sure," cried Catherine, "I did not mean to say anything wrong; but it is a nice book, and why should I not call it so?" "Very true," said Henry, "and this is a very nice day, and we are taking a very nice walk; and you are two very nice young ladies. Oh! It is a very nice word indeed! It does for everything." [Jane Austen, "Northanger Abbey," 1803]

For sense evolution, compare fond, innocent, lewd, also silly, simple.

Middle English pratie "cunning, crafty, clever" (c. 1300 as a surname), from Old English prættig (West Saxon), pretti (Kentish), *prettig (Mercian) "cunning, skillful, artful, wily, astute," from prætt, *prett "a trick, wile, craft," from Proto-Germanic *pratt- (source also of Old Norse prettr "a trick," prettugr "tricky;" Frisian pret, Middle Dutch perte, Dutch pret "trick, joke," Dutch prettig "sportive, funny," Flemish pertig "brisk, clever"), a word of unknown origin.

The connection between the Old English and Middle English words "has several points of obscurity" [OED], and except in surnames there is no record of it 13c.-14c., but they generally are considered the same. The meaning had expanded by c. 1400 to "manly, gallant," also "ingeniously or cleverly made," to "fine, pleasing to the aesthetic sense," to "beautiful in a slight way" (mid-15c.). Also used of bees (c. 1400). For sense evolution, compare nice, silly, neat (adj.), fair (adj.).

Pretty applies to that which has symmetry and delicacy, a diminutive beauty, without the higher qualities of gracefulness, dignity, feeling, purpose, etc. A thing not small of its kind may be called pretty if it is of little dignity or consequence: as a pretty dress or shade of color; but pretty is not used of men or their belongings, except in contempt. [Century Dictionary, 1897]

Of things, "fine, pleasing" 1560s. Ironical use is from 1530s (compare ironical use of fine (adj.)). The meaning "not a few, considerable, moderately large in quantity, number, extent, or duration" is from late 15c. Pretty please as an emphatic plea is attested from 1902. A pretty penny "lot of money" is recorded from 1703.

"in a foolish or absurd manner," 1620s, from silly + -ly (2).

"foolishness, senselessness, weakness of understanding, absurd folly," c. 1600, from silly + -ness; a reformation of earlier seeliness, from Old English saelignes "happiness, (good) fortune, occurrence," which was formed from the same elements.

c. 1200, "free from duplicity, upright, guileless; blameless, innocently harmless," also "ignorant, uneducated; unsophisticated; simple-minded, foolish," also as a surname, from Old French simple (12c.) "plain, decent; friendly, sweet; naive, foolish, stupid," hence also "wretched, miserable," from Latin simplus or simplex, "simple, plain, unmixed," literally "one-fold" (see simplex).

The sense evolution is from the notion of "without parts" or "having few parts," hence "free from complexity or complication." Compare the similar sense evolution of silly. The extended senses in Latin simplex were "without dissimulation, open, frank, guileless, direct, ingenuous," sometimes "too straightforward, too blunt," but Latin seems not to have had the "simple-minded" meaning.

The sense of "free from pride, humble, meek" is from mid-13c. As "consisting of only one substance or ingredient" (opposite of composite or compounded) it dates from late 14c.; as "easily done, presenting no difficulty or obstacles" (opposite of complicated) it dates from late 15c.; that sense also was in Latin.

From mid-14c. as "unqualified; mere; sheer," a sense also found in Latin; also "clear, straightforward; easily understood." From late 14c. as "single, individual; whole." From late 14c. of clothing, etc., "modest, plain, unadorned," and of food, "plain, not sumptuous." In medicine, of fractures, etc., "lacking complications," late 14c. As a law term, "lacking additional legal stipulations, unlimited," from mid-14c.

The Middle English word had senses that have been lost, including "inadequate, insufficient; weak, feeble; mere; few; sad, downcast; mournful; of little value; low in price; impoverished, destitute," and, of hair, "straight, not curly."