A W Moore was of the following opinion - the name Costean has been found in Mann since the earliest records:
COSTAIN and COSTEAN, contracted from MacAusteyn, a shortened form of MacAugustin, 'Augustin's son.' (Augustin is the diminative of Augustus). The fame of AUGUSTINUS of Hippo, and his namesake, the missionary of the English, would cause this name to be a favourite among Christian converts.

Magnus Barfod, King of Norway, who died A.D. 1103, had a son OSTEEN and a grandson, son of Harold Gyllie, OSTEN.

COSTAIN and COSTEAN are purely Manx names.

COSTEANE [1407], MACCOISTEN, MACCOSTEN, COISTEN, COSTEN [1511], CAUSTEEN [1687], COSTAIN [1714], COSTEAN [1747].