What is the X in X-mas?
c.1400, from Latin impedimentem "hindrance," from impedire "impede," literally "to shackle the feet," from assimilated form of in- "into, in" (see in- (2)) + pes (genitive pedis) "foot" (see foot (n.)).
impediment im·ped·i·ment (ĭm-pěd'ə-mənt)n.
Something that impedes; a hindrance or an obstruction.
An organic defect preventing clear articulation of speech.