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. Proceeded 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 to hit the spot "satisfy, be what is required" is from 1868. Spot check first attested 1933. Adverbial phrase spot on "completely right" attested from 1920.