Origin and history of spotty

c. 1200, "moral stain;" by mid-14c. as "speck, stain left by something on a surface;" probably at least in part from a variant of Old English splott "a spot, blot, patch (of land)," and partly from or related to Middle Dutch spotte "spot, speck." Other cognates are East Frisian spot "speck," North Frisian spot "speck, piece of ground," Old Norse spotti "small piece," Norwegian spot "spot, small piece of land." Likely some of these Germanic words are borrowings of some of the others, but the exact evolution is unclear.

From c. 1300 as "patch or mark on the fur of an animal." The sense of "particular place, small extent of space" (on a body, etc.) is from late 14c. In general figurative use, "a blemish, defect, distinguishing mark," late 14c. Also from late 14c. as "an eruption on the skin."

The meaning "short interval in a broadcast for an advertisement or announcement" is by 1937, from earlier sense of "an act's position on a bill" 1923. Preceded by a number (as in five-spot) it originally was a term for "prison sentence" of so many years (1901, American English slang). The sense in night-spot is by 1954.

Colloquial phrase hit the spot "satisfy, be what is required" is by 1857. The adverbial phrase on the spot is attested by 1670s as "at once, without moving or delay;" 1680s as "in the precise place and time." Hence to be on the spot "doing just what is right and needed" (1884). To put (someone) on the spot "place in a difficult situation" is from 1928; to be in a spot "in difficulty" is by 1929. Spot check, made on a random sample, is attested by 1933; as a verb by 1944. Adverbial phrase spot on "completely right" attested from 1920.

very common adjective suffix, "full of, covered with, or characterized by" the thing expressed by the noun, Middle English -i, from Old English -ig, from Proto-Germanic *-iga-, from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic). Germanic cognates include Dutch, Danish, German -ig, Gothic -egs.

It was used from 13c. with verbs (drowsy, clingy), and by 15c. with other adjectives (crispy). Chiefly with monosyllables; with more than two the effect tends to become comedic.

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Variant forms in -y for short, common adjectives (vasty, hugy) helped poets after the loss of grammatically empty but metrically useful -e in late Middle English. Verse-writers adapted to -y forms, often artfully, as in Sackville's "The wide waste places, and the hugy plain." (and the huge plain would have been a metrical balk).

After Coleridge's criticism of it as archaic artifice, poets gave up stilly (Moore probably was last to make it work, with "Oft in the Stilly Night"), paly (which Keats and Coleridge himself had used) and the rest.

Jespersen ("Modern English Grammar," 1954) also lists bleaky (Dryden), bluey, greeny, and other color words, lanky, plumpy, stouty, and the slang rummy. Vasty survives, he writes, only in imitation of Shakespeare; cooly and moisty (Chaucer, hence Spenser) he regards as fully obsolete. But in a few cases he notes (haughty, dusky) they seem to have supplanted shorter forms.