1540, abbreviation of videlicet "that is to say, to wit, namely" (1464), from L. videlicet, contraction of videre licet "it is permissible to see," from videre "to see" (see vision
) + licet "it is allowed," third person singular present indicative of licere "be allowed."
The -z- is not a letter, but originally a twirl, representing the usual M.L. shorthand symbol for the ending -et. "In reading aloud usually rendered by 'namely.' " [OED]