Casson, English, and according to Reaney's surname dictionary, can mean "son of Catt" (Catherine?), or "son of Cass", short for Cassandra, so a metronym. Cassin is both Irish and Manx, i.e., Gaelic; MacCaisin originally, "son of Caisin" a man's name which incorporates the word cas, crooked." There's also a French surname Cassin, presumably from a place-name. It derives from an old Celtic word for an oak tree, as do the Italian names, Cassano and Cassani.