What is the X in X-mas?
early 15c., "genealogical table or chart," from Anglo-French pe de gru, a variant of Old French pied de gru "foot of a crane," from Latin pedem accusative of pes "foot" (see foot (n.)) + gruem (nominative grus) "crane," cognate with Greek geranos, Old English cran; see crane (n.)).
On old manuscripts, "descent" was indicated by a forked sign resembling the branching lines of a genealogical chart; the sign also happened to look like a bird's footprint. Form influenced in Middle English by association with degree. Meaning "ancestral line" is mid-15c.; of animals, c.1600. Related: Pedigreed.
pedigree ped·i·gree (pěd'ĭ-grē')
An ancestral line of descent, especially as diagrammed on a chart, to show ancestral history and to analyze Mendelian inheritance of certain traits including familial diseases.