What is the X in X-mas?
"speculative," mid-15c., from French présomptif (15c.), from Medieval Latin presumptivus, from Late Latin praesumptivus, from Latin praesumpt- past participle stem of praesumere (see presume). The heir presumptive (1620s) is "presumed" to be the heir if the heir apparent is unavailable. Related: Presumptively.