What does Boxing Day have to do with boxing?
"of or pertaining to Siam," 1690s; see Siam + -ese. Also from 1690s as a noun meaning "native of Siam." the original Siamese twins (exhibited from 1829) were Chang and Eng (1814-1874), Thai-Chinese natives of Siam who settled in the U.S. Hence Siamesed (adj.) "joined in the manner of Siamese twins" (1830). Siamese cat is attested from 1871.