What is the X in X-mas?
early 15c., from Middle French acuité (16c.) or directly from Medieval Latin acuitatem (nominative acuitas) "sharpness," from Latin acuere "to sharpen," related to acus "needle," acuere "to sharpen," from PIE root *ak- "rise to a point, be sharp" (see acrid).
acuity a·cu·i·ty (ə-kyōō'ĭ-tē)
Sharpness, clearness, and distinctness of perception or vision.