Old English dott "speck, head of a boil," perhaps related to Norwegian dot "lump, small knot," Dutch dot "knot, small bunch, wisp," Old High German tutta "nipple;" ultimate origin unclear.
Known from a single source c.1000; the word reappeared with modern meaning "mark" c.1530; not common until 18c. Morse telegraph sense is from 1838. On the dot "punctual" is 1909, in reference to a clock dial face. Dot-matrix first attested 1975.