What is the X in X-mas?
1840, from biology + -ical. Biological clock attested from 1955; not especially of human reproductive urges until c.1991. Related: Biologically.
biological bi·o·log·i·cal (bī'ə-lŏj'ĭ-kəl)adj.
Of, relating to, caused by, or affecting life or living organisms.
Having to do with biology.
Related by blood, as in a child's biological parents.