What is the X in X-mas?
early 15c., "action of pressing out;" later (mid-15c.) "action of manifesting a feeling;" (late 15c.) "a putting into words," from Middle French expression (14c.), from Late Latin expressionem (nominative expressio), noun of action from past participle stem of exprimere (see express (v.)). Meaning "an action or creation that expresses feelings" is from 1620s. Of the face, from 1774. Occasionally the word also was used literally, for "the action of squeezing out."
expression ex·pres·sion (ĭk-sprěsh'ən)
The act of pressing or squeezing out.
The outward manifestation of a mood or disposition by mobility of the facial features; facies.
The phenotype manifested by a genotype under fixed environmental conditions.