The root appears to stem from Basque semen ('son'), attested in Aquitanian inscriptions as Sembeconnis and like forms.
Variants of the surname include Jimenes, Ximénez/Ximenes, Giménez/Gimenes, Chiménez, Chimenes, Seménez, Semenes, Ximenis or Eiximenis in Catalonia, in Sicilian Scimemi or Scimeni and the Neapolitan Chimenz or Chimenez.
As the modern name Ximenes has an -es suffix, it is almost certainly of Portuguese, Galician or Old Spanish origin, as the orthographic change to -ez (and the consonant shift from X to J) was revised in Spain only in the late 18th century.
Other languages in Castilian-dominated lands like Aragon, Asturias, Galicia, often retained the -es ending, and their descendants bear witness to this historical anomaly.
In Catalonia, Valencia and the Balearics, the ending -is is used instead of -es (or -ez), hence, the spelling Ximenis (or the variant with vowel epenthesis, Eiximinis or Eximenis).