What is the X in X-mas?
pertaining to the chin
Greek geneion 'chin'
c.1600, "festivity;" 1650s, "cheerfulness," from Late Latin genialitas "festivity, pleasantness," from Latin genialis (see genial).
1560s, "pertaining to marriage," from Latin genialis "pleasant, festive," literally "pertaining to marriage rites," from genius "guardian spirit" (see genius). Originally used in the Latin literal sense; meaning "cheerful, friendly" first recorded 1746. Related: Genially.
genial ge·ni·al (jĭ-nī'əl) or ge·ni·an (-ən)
Of or relating to the chin.