What is the X in X-mas?
late 13c., auys "opinion," from Old French avis "opinion, view, judgment, idea" (13c.), from phrase ço m'est à vis "it seems to me," or from Vulgar Latin *mi est visum "in my view," ultimately from Latin visum, neuter past participle of videre "to see" (see vision).
The unhistoric -d- was introduced in English 15c., on model of Latin words in ad-. Substitution of -c- for -s- is 18c., to preserve the breath sound and to distinguish from advise. Meaning "opinion given as to action, counsel" is from late 14c.