Cassar is a common Maltese surname.
There are various possible origins of the surname.
It may be traced back to the Kingdom of the Two Sicilies, incorporating the island of Sicily itself, the area around Naples, and the southern part of the Italian peninsula. The surname Cassar may have derived from the given name "Cesare," and further back from the Roman family name "Caesar." In Classical antiquity, the name "Caesar" was associated by folk etymology with the Latin word "caesaries" (meaning head of hair). The Maltese Cassar coat of arms has the Latin word "SPES" (meaning "hope") inscribed on it.
On the other hand it may derive from a fairly common surname in Italy and Sicily: Cassara, or Cassarino, possibly from a nickname "cassaio" meaning "producer of cases".
Others suggest that it originates from Spain under Moorish rule. The Arabic surname Al-Qassar means castle, from the Arabic "al-qasr" (palace, fortress or mansion), compare the Spanish "alcazar" (fortress). However the Spanish version of the surname is 'Casar' not 'Cassar'. One version of the coat of arms is a fortified tower with a paschal lamb on top holding a flag.
From about 800 A.D. to 1090 A.D. both Sicily and Malta were occupied by the Arabs. Control was varyingly exercised from Tunisia (Sunni Aghlabids) and Egypt (Shiite Fatimids). Arab and Muslim culture lingered until the 13th century. Many common Maltese surnames are of Arabic origin, such as Xuereb, Axiaq, Asciak, Abdilla. Casha (or Qasha), Qassar (or Kassar) are common north African surnames.
Another possibility is that it comes from the Maltese word "qassar" (barber, literally "a person who cuts short") from "qasir" (short). In old Maltese "q" was pronounced as "q".