before 1000; perhaps to be identified with Old Englishdott head of a boil, though not attested in Middle English; cf. dottle, dit, derivative of Old Englishdyttan to stop up (probably derivative of dott); cognate with Old High Germantutta nipple
O.E. dott "speck, head of a boil," perhaps related to tit "nipple." 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.
Exactly on time, as in We had to be there at eight on the dot. The dot in this idiom is the mark appearing on the face of a watch or clock indicating the time in question. It may come from the earlier to a dot, meaning “exactly” since the early 1700s but no longer heard today.
[ c. 1900