mid-14c., "marked with spots," from spot (n.) + -y (2). Meaning "unsteady, uneven" is attested from 1932, from a more specific use with reference to painting (1812).
Entries linking to spotty
spot n.
c. 1200, "moral stain," probably from 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." It is likely that some of these are borrowed from others, but the exact evolution now is impossible to trace.
Meaning "speck, stain" is from mid-14c. The sense of "particular place, small extent of space" is from c. 1300. Meaning "short interval in a broadcast for an advertisement or announcement" is from 1923. Preceded by a number (as in five-spot) it originally was a term for "prison sentence" of that many years (1901, American English slang). To put (someone) on the spot "place in a difficult situation" is from 1928. Colloquial phrase hit the spot "satisfy, be what is required" is from 1868. Spot check is attested by 1933. Adverbial phrase spot on "completely right" attested from 1920.
-y 2
adjective suffix, "full of or characterized by," from Old English -ig, from Proto-Germanic *-iga- (source also of Dutch, Danish, German -ig, Gothic -egs), from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic). Originally added to nouns in Old English; used from 13c. with verbs, and by 15c. even with other adjectives (for example crispy). Adjectives such as hugy, vasty are artificial words that exist for the sake of poetical metrics.