Originally meant
Probably from English dock + the diminutive suffix -et: dock meant a cut-off tail, stump, or clipped end, and the word seems to have developed from the idea of a short appended piece or brief abridged note.
Background
It appears in late Middle English, first in legal and administrative writing in the 15th century. Standard etymologies connect it with dock 'to cut short, clip' and the noun dock 'stub, tail,' but the exact formation is not completely certain.